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Subphylum Vertebrata (Sponges)

Sponges are a diverse group of sometimes common types, with about 5000 species known across the world. Sponges are primarily marine, but around 150 species live in fresh water. Sponges have cellular-level organization, meaning that that their cells are specialized so that different cells perform different functions, but similar cells are not organized into tissues and bodies are a sort of loose aggregation of different kinds of cells. This is the simplest kind of cellular organization found among parazoans.

Other characteristics of sponges include a system of pores (also called ostia) and canals, through which water passes. Water movement is driven by the beating of flagellae, which are located on specialized cells called choanocytes (collar cells). Sponges are either radially symmetrical or asymmetrical. They are supported by a skeleton made up of the protein collagen and spicules, which may be calcareous or siliceous, depending on the group of sponges examined. Skeletal elements, choanocytes, and other cells are imbedded in a gelatinous matrix called mesohyl or mesoglea. Sponges capture food (detritus particles, plankton, bacteria) that is brought close by water currents created by the choanocytes. Food items are taken into individual cells by phagocytosis, and digestion occurs within individual cells.

Reproduction by sponges is by both sexual and asexual means. Asexual reproduction is by means of external buds. Some species also form internal buds, called gemmules, which can survive extremely unfavorable conditions that cause the rest of the sponge to die. Sexual reproduction takes place in the mesohyl. Male gametes are released into the water by a sponge and taken into the pore systems of its neighbors in the same way as food items. Spermatozoa are "captured" by collar cells, which then lose their collars and transform into specialized, amoeba-like cells that carry the spermatozoa to the eggs. Some sponges are monoecious; others are dioecious. In most sponges for which developmental patterns are known, the fertilized egg develops into a blastula, which is released into the water (in some species, release takes place right after fertilization; in others, it is delayed and some development takes place within the parent). The larvae may settle directly and transform into adult sponges, or they may be planktonic for a time. Adult sponges are always sessile.

Sponges fall into three main groups according to how their bodies are organized. The simplest sponges are the asconoid sponges. These are shaped like a simple tube perforated by pores. The open internal part of the tube is called the spongocoel; it contains the collar cells. There is a single opening to the outside, the osculum. The next-most complicated group is the syconoids. These tend to be larger than asconoids. They also have a tubular body with a single osculum, but their body wall is thicker and the pores that penetrate it are longer, forming a system of simple canals. These canals are lined by collar cells, the flagellae of which move water from the outside, into the spongocoel and out the osculum. The third category of body organization is leuconoid. These are the largest and most complex sponges. These sponges are made up of masses of tissue penetrated by numerous canals. Canals lead to numerous small chambers lined with flagellated cells. Water moves through the canals, into these chambers, and out via a central canal and osculum.

Sponges are found in virtually all aquatic habitats, although they are most common and diverse in the marine environment. Many species contain toxic substances, probably to discourage predators. Certain other marine animals take advantage of this characteristic of sponges by placing adult sponges on their bodies, where the sponges attach and grow. The chemicals also probably play a role in competition among sponges and other organisms, as they are released by sponges to insure themselves space in the marine ecosystem. Some of these chemicals have been found to have beneficial pharmaceutical effects for humans, including compounds with respiratory, cardiovascular, gastrointestinal, anti-inflammatory, antitumor, and antibiotic activities. Sponges also provide a home for a number of small marine plants, which live in and around their pore systems. Symbiotic relationships with bacteria and algae have also been reported, in which the sponge provides its symbiont with support and protection and the symbiont provides the sponge with food. Some sponges (boring sponges) excavate the surface of corals and molluscs, sometimes causing significant degradation of reefs and death of the mollusc. The corals or molluscs are not eaten; rather, the sponge is probably seeking protection for itself by sinking into the hard structures it erodes. Even this process has some beneficial effects, however, in that it is an important part of the process by which calcium is recycled.


Phylum Cnidaria

The Phylum Cnidaria includes such diverse forms as jellyfish, hydra, sea anemones, and corals. Cnidarians are radially or biradially symmetric, a general type of symmetry believed primitive for eumetazoans. They have achieved the tissue level of organization, in which some similar cells are associated into groups or aggregations called tissues, but true organs do not occur. Cnidarian bodies have two or sometimes three layers. A gastrovascular cavity (coelenteron) has a single exterior opening that serves as both mouth and anus. Often tentacles surround the opening. Some cells are organized into two simple nerve nets, one epidermal and the other gastrodermal, that help coordinate muscular and sensory functions.
Cnidarians have two basic body forms, medusa and polyp. Medusae, such as adult jellyfish, are free-swimming or floating. They usually have umbrella-shaped bodies and tetramerous (four-part) symmetry. The mouth is usually on the concave side, and the tentacles originate on the rim of the umbrella.


Class Hydrozoa

These organisms may exist as either polyps or medusae. Many species pass through both forms in their life cycles; in others, one form or the other is suppressed. The majority of species are marine and colonial, but many species are solitary and some live in fresh water. Hydrozoans include such diverse forms as hydra, which are a group of species of solitary, fresh-water forms that live as polyps throughout their lives. Reproduction by hydras can be the typical form for polyps, asexual by budding, or by sexual means in which temporary gonads appear on the polyp itself.

Hydrozoans also include colonial species, called hydroids. Hydroids are actually colonies of polyps growing on a common stalk. Colonies are formed by the asexual budding of members; these buds, unlike those of hydras, remain attached to the parent. Some polyps in a typical hydroid specialize in reproduction; these produce medusae by budding. In this typical case, the medusae, which are usually free-swimming, leave the colony, mature, and produce gametes for sexual reproduction. The gametes are shed, fertilization takes place, and the zygote develops into a free-swimming, ciliated planula larva. After a time the planula settles to the bottom and develops into a polyp, which by budding eventually forms a new colony. Not all hydroids follow this pattern; in some, the medusa phase is compressed or reduced and gametes are shed directly from specialized polyps.

Some hydrozoans, such as the Portuguese man-of-war, form floating colonies that include both specialized medusae and polyps. Others, called hydrocorals, have calcareous skeletons that resemble those of true corals.

Hydrozoan medusae all have a distinctive structure called a velum. This is a shelf or rim that projects inward around the margin of the bell, partially closing the opening. Hydrozoan medusae swim by alternately constricting and relaxing muscles in the bell. This causes water to shoot out the constricted opening of the bell, moving the animal by a sort of "jet propulsion."


Class Scyphozoa

The Class Scyphozoa includes most of the larger jellyfish. The medusae of members of this class lack a velum. In many species, the rim of the bell contains sense organs, including statocysts that sense balance and orientation and photoreceptors that are sensitive to light. Scyphozoans are often amply covered with nematocysts, so that a swimmer's encounter with one can be an unpleasant experience. They feed in a variety of ways, but often involving prey capture by nematocysts on the arms and transport of food items to the gastrovascular cavity by means of cilia. As in other cnidarians, digestion is intracellular. Scyphozoans are frequently strikingly tetramerous (four-parted). Four gastric pouches connect with the gastrovascular cavity, and the opening to that cavity may have four lobes. The peripheral sense organs are usually placed in pits or notches; these are often present in multiples of four.

Scyphozoans are dioecious. Fertilization and early development usually take place in the gastrovascular cavity or on the lobes near the gastrovascular opening. A ciliated planula larva forms; in most scyphozoans it settles and forms a hydra-like polyp. This life stage, called a scyphistoma, reproduces asexually by budding. At first the product of budding is other scyphistomas, but later they produce tiny medusae that break free and grow to form mature, sexual scyphozoan medusae.


Class Anthozoa

The Class Anthozoa includes a variety of animals that have polyps with a flower-like appearance. In these forms, the gastrovascular cavity is large. It is divided by walls or septa, which arise as folds from the body wall. These folds, along with the mouth and pharynx, are usually arranged in a biradially symmetric pattern.

Anthozoans include sea anemones, a variety of corals, sea fans, and sea pens. Sea anemones are carnivorous polyps that are quite large, ranging up to 200mm in length. They tend to be brightly colored. Most species live in warm water. They feed on fishes, which are caught by means of the numerous nematocysts in their tentacles. These animals are known for their symbionts. These include species of fish that actually live among the tentacles of large anemones, somehow avoiding lethal contact with the nematocysts. Other anemones have unicellular algae living within their tissues, from which they probably derive some nutrition. Yet others have a symbiotic relationship with hermit crabs, which gather up the anemones and place them on the snail shells that the crabs occupy. The anemones benefit from food particles dropped by the crab, and the crab gains protection from predators due to the presence of the nematocyst-laden anemones.

The Class Anthozoa also includes many kinds of corals, including many reef-building species. Reefs are formed by the calcareous skeletons of many generations of coral polyps. The polyps inhabit only the surface of the reefs. These reefs are among the most productive environments of the world, housing thousands of species of fish and invertebrates, not to mention plants and protists. Like some anemones, corals are inhabited by symbiotic algae called zooxanthellae. These photosynthetic algae are essential for the coral, and corals generally do not live at depths to which light does not penetrate.


Phylum Phatyhelminthes

Flatworms are unsegmented, bilaterally symmetrical worms that lack a coelom (acoelomate) but that do have three germ layers. Some forms are free living but many are parasitic. Flatworms have a cephalized nervous system that consists of head ganglion, usually attached to longitudinal nerve cords that are interconnected across the body by transverse branches. Excretion and osmoregulation by flatworms is controlled by "flame cells" located in protonephridia (thse are absent in some forms). Flatworms lack a respiratory or circulatory system; these functions take place by absorption through the body wall. Nonparasitic forms have a simple, incomplete gut; even this is lacking in many parasitic species. Movement in some flatworms is controlled by longitudinal, circular, and oblique layers of muscle. Others move along slime trails by the beating of epidermal cilia. The development of directional movement is correlated with cephalization. In some flatworms, the process of cephalization has included the development in the head region of light-sensitive organs called ocelli. Other sense organs found in at least some members of this group (not necessarily on the head) include chemoreceptors, balance receptors (statocysts), and receptors that sense water movement (rheoreceptors).

Most flatworms can reproduce sexually or asexually. Most are monoecious. Most of these have developed ways of avoiding self-fertilization. Development may be direct (eggs hatch into tiny worms that resemble the adults) or indirect (with a ciliated larval form).

Flatworms include a large number of parasitic forms, some of which are extremely damaging to human populations.


Class Turbellaria

Turbellarians are predominately free-living and aquatic. They have an incomplete digestive tract, in which the mouth leads to a pharynx, then to temporary spaces containing cells that take in food particles by phagocytosis. Digestion is intracellular. These animals move by laying down slime from special skin glands, then gliding along these "slime trails" by the beating of epidermal cilia.

Turbellarians are mostly carnivorous, preying on tiny invertebrates that they locate by means of their chemoreceptors. They are a diverse group, including over 4500 known species divided into 12 Orders.


Class Trematoda

All trematodes are parasitic, and most adult trematodes parasitize vertebrates. Around 9000 species have been described. Their body is covered with a tegument, a peculiar kind of epidermal arrangement in which the main cell bodies are deep, separated from the cytoplasm that lies next to the exterior by a layer of muscle (but connected to the exterior layer by cellular processes. The exterior layer is syncytial; that is, it is continuous, not broken by cell membranes. The tegument lacks cilia in adults. Unlike monogeneans, trematodes have no opisthoaptor; instead, they are characterized by one or two suckers. They are like turbellarians in having a relatively well developed alimentary canal, and their muscular, excretory, and reproductive systems are also relatively complete.

Most trematodes have complex life cycles, with larval stages parasitizing one or more species that are different from host of adults. Larval stages of some medically important species include miracidium, redia, cercaria, and metacercaria. Most trematodes are endoparasites. They include several parasites that have an enormous impact on human populations, such as human liver flukes and the blood flukes that cause schistosomiasis.


Class Cestoda

The cestodes, or tapeworms, differ in a number of ways from other flatworms. Their bodies are long and flat, made up of many segments called proglottids. Each proglottid is a reproductive unit, essentially a factory to produce gametes. Adults lack cilia and their surface is a tegument (as in monogeneans and trematodes), but in cestodes the tegument is covered with tiny projections, microvilli, which increase its surface area and thereby its ability to absorb nutrients from a host. Digestive tracts are absent completely. At the tapeworm's anterior end is a specialized segment called a scolex, which is usually covered with hooks or suckers and serves to anchor it to the host.

All of the 5000 or so known species of tapeworms are endoparasites. Most require at least two hosts, with the host of the adult tapeworm a vertebrate. Intermediate hosts are often invertebrates. A number of tapeworm species inhabit humans.


Phylum Nematoda

Roundworms (nematodes) are bilaterally symmetrical, worm-like organisms that are surrounded by a strong, flexible noncellular layer called a cuticle. Their body plan is simple. The cuticle is secreted by and covers a layer of epidermal cells. Near the body wall but under the epidermal cells are muscle cells; they run in the longitudinal direction only. A true coelom is lacking, instead, nematodes have a "pseudocoel" formed directly from the cavity of the blastula (rather than as a result of the division or folding of mesoderm). The cavity of the pseudocoel is small, being mostly filled with an intestine and oviducts or testes. A simple nervous system consists of a ring of nervous tissue around the pharynx that gives rise to dorsal and ventral nerve cords running the length of the body.

Nematodes move by contraction of the longitudinal muscles. Because their internal pressure is high, this causes the body to flex rather than flatten, and the animal moves by thrashing back and forth. No cilia or flagellae are present.

Some nematodes have specialized cells that excrete nitrogenous wastes; in others, canals or canals plus these specialized cells are present. Nematodes do not have flame cells.

Most nematodes are dioecious. Fertilization takes place when males use special copulatory spines to open the females' reproductive tracts and inject sperm into them. The sperm are unique in that they lack flagellae and move by pseudopodia, like amoebas. Development of fertilized eggs is usually direct.

Nematodes are almost unbelievably abundant. One study reported around 90,000 individual nematodes in a single rotting apple. Another reported 236 species living in a few cubic centimeters of mud. The number of described species is around 12,000, but too little attention has been paid to these animals and the true number may be closer to 500,000. Some species are generalists, occuring across wide areas and in many habitats; others are much more specialized. A good example of the latter is a species of nematode that is known only from felt coasters placed under beer mugs in a few towns in Germany.

Many nematodes are free living and play critical ecological roles as decomposers and predators on microorganisms. But nematodes also include parasitic species, a number of which affect humans directly or indirectly through their domestic animals. These include the common roundworms, which probably infest more than half the world's humans; hookworms; trichina, the worms that cause trichinosis; pinworms, another extremely common parasite, even in the United States, which can be transmitted from human to human by eggs floating in household dust; and filarial worms, primarily tropical parasites that cause diseases such as filariasis (elephantiasis) and onchocerciasis (river blindness).


Phylum Mollusca

Molluscs are bilaterally symmetrical eumetazoans. They have a true coelom (eucoelom) which is formed by the splitting of embryonic mesodermal masses (schizocoely). Development is protostomous. An important characteristic of most molluscs is the head-foot region. Most molluscs are strongly cephalized; that is, they have a well-developed head, in which is located a mouth and a concentration of nervous and sensory functions. Adjacent to the head is a large, muscular foot formed from the ventral body wall. Used primarily in locomotion, the surface of the foot is sometimes ciliated and and laden with numerous mucous glands.

Another characteristic of most molluscs is the mantle. This sheath of tissue is formed from the dorsal body wall. It surrounds the mantle cavity, which houses the gills or lungs if they are present, and its surface may assist in gas exchange. The mantle also secretes the shell in those forms that possess one.

Internally, molluscs have a complicated digestive system, with a mouth in the head and the anus emptying into the mantle cavity. A rasping organ used in feeding, the radula, is present in all groups except bivalves and Aplacophora. The radula is usually toothed and its structure may be very complex. It is adapted to a wide variety of feeding styles in different species, including scraping, tearing, stabbing, and cutting.

Molluscs have an open circulatory system (except cephalopods, in which it is closed), complete with a heart, blood vessels, and respiratory pigments. Gas exchange is via gills, lungs, or the body surface. Excretion takes place by means of kidneys that, like the digestive tract, pump waste into the mantle cavity.

A fairly complicated nervous system is present, including several ganglia and a system of nerves; sensory and nervous systems are cephalized in at least some kinds of molluscs. Some molluscs have complex, extremely sensitive eyes.

The majority of molluscs have a shell of some kind. This calcareous structure is secreted by the mantle, and in living snails it is covered by a horny layer called a periostracum. Its shape, size, and color are widely used by taxonomists, and they has also made molluscs a popular pursuit of collectors. Further, shells decay slowly and fossilize well, and the fossil record of molluscs is excellent and ancient.

Most molluscs are dioecious. Many pass through free-swimming larval stages, called trochophore and veliger larvae.

Molluscs are an extremely diverse group of organisms. Over 50,000 living species have been named, making Mollusca second only to the Phylum Arthropoda in number of species known. The majority of molluscs are marine, but large numbers of species occupy freshwater and terrestrial habitats. Molluscs are also extremely diverse in their food habits, ranging from species that graze on microscopic algae, to those that eat the leaves of terrestrial plants, to predators that capture fish and other molluscs. Many species of molluscs are important to humans. A large number of bivalves and some snails are important sources of protein. Oysters produce pearls. Other species are pests in gardens and to crops. Some are essential components in the life cycles of human parasites, including devastating diseases such as schistosomiasis. And a few, such as oyster drills, cause problems by preying on other molluscs that are important to us.

Finally, molluscs play a wide variety of essential ecological roles. As common herbivores, they can have a significant impact on the plant species present in an area. As predators, they may have a similar effect on animals, especially other molluscs. As prey, they provide food for a large number of organisms, including many vertebrates (some of which, such as sea otters, are strikingly specialized for feeding on them). An example of the ecological (not to mention economic) impact of molluscs is given by the recent introduction of zebra mussels to the Great Lakes. Zebra mussels live in large numbers on hard surfaces, and they feed by filtering particles from the water. The effectiveness of their feeding is remarkable; water clarity in some of the Great Lakes has improved dramatically in the years since zebra mussels first appeared. Unfortunately, the same material that the mussels are efficiently removing is the basis of a complicated food chain for other species, including economically-important game fishes. Fisheries biologists in the Great Lakes states are very concerned about the future of the species they tend. The impact of these changes on less conspicuous species, such as the many kinds of native bivalves, is not known, but their populations are certainly at risk.


Class Gastropoda

The Class Gastropoda includes the snails and slugs. Most gastropods have a single, usually spirally coiled shell into which the body can be withdrawn, but the shell is lost or reduced some important groups. Gastropods are characterized by "torsion," a process that results in the rotation of the visceral mass and mantle on the foot. The result is that the mantle cavity (including anus) lies in the anterior body, over the head and mouth, and the gut and nervous system are twisted. Torsion takes place during the veliger stage, usually very rapidly. Veligers are at first bilaterally symmetric, but torsion destroys this pattern and results in an asymmetric adult. Some species reverse torsion ("detorsion"), but evidence of having passed through a twisted phase can be seen in the anatomy of these forms. Many snails have an operculum, a horny plate that seals the opening when the snail's body is drawn into the shell. Gastropods have a muscular foot which is used for "creeping" locomotion in most species. In some, it is modified for swimming or burrowing. Most gastropods have a well-developed head that includes eyes, 1-2 pairs of tentacles, and a concentration of nervous tissue (ganglion).

Gastropods are dioecious, and some forms are hermaphroditic. Hermaphroditic forms exchange bundles of sperm to avoid self-fertilization; copulation may be complex and in some species ends with each individual sending a sperm-containing dart into the tissues of the other. Marine species have veliger larvae.

Torsion in gastropods has the unfortunate result that wastes are expelled from the gut and nephridia near the gills. A variety of morphological and physiological adaptations have arisen to separate water used for respiration from water bearing waste products.

Gastropods are by far the largest group of molluscs. Their 40,000 species comprise over 80% of living molluscs. Gastropod feeding habits are extremely varied, although most species make use of a radula in some aspect of their feeding behavior. Some graze, some browse, some feed on plankton, some are scavengers or detritivores, some are active carnivores.


Class Cephalopoda

This very diverse group includes octopus, squid, nautilus, and cuttlefish. A shell is sometimes present. If present, it is divided into chambers, and the animal occupies the last-formed chamber, with a filament of tissue extending to the oldest chamber. The foot of cephalopods, as their name suggests, lies close to the head. It is modified to form a funnel, through which water is expelled from the mantle cavity. This sort of jet propulsion results in very fast locomotion in some species. Around their mouths, cephalopods have a ring of tentacles that bear powerful suckers in some species. The tentacles are used to search out and grasp food. Most cephalopods have chromatophores, special pigment cells, in their skins, which allow them to change color rapidly. Most also have a sac that secretes an ink-like substance called sepia; this is expelled when the animal is alarmed, providing concealment as the animal flees.

Cephalopods have the most complex brain of any invertebrate. Most species also have excellent eyes that are astonishingly similar to those of vertebrates. Most species have a single pair of gills, a mostly-closed circulatory system, and accessory or branchial hearts. The gills are ventilated by the muscular action of the mantle wall.

Cephalopods are dioecious. They copulate when the male uses one of his arms (specially modified for reproduction in some species) to remove a spermatophore from his mantle cavity and place it in the mantle cavity of a female. Fertilized eggs are attached to the substrate and are sometimes tended by the female.


Phylum Annelida

The annelids include earthworms, polychaete worms, and leeches. All members of the group are to some extent segmented, in other words, made up of segments that are formed by subdivisions that partially transect the body cavity. Segmentation is also called metamerism. Segments each contain elements of such body systems as circulatory, nervous, and excretory tracts. Metamerism increases the efficiency of body movement by allowing the effect of muscle contraction to be extremely localized, and it makes possible the development of greater complexity in general body organization. Besides being segmented, the body wall of annelids is characterized by being made up of both circular and longitudinal muscle fibers surrounded by a moist, acellular cuticle that is secreted by an epidermal epithelium. All annelids except leeches also have chitonous hair-like structures, called setae, projecting from their cuticle. Sometimes the setae are located on paddle-like appendages called parapodia.

Annelids are schizocoelous and with a large and well-developed true coelom (i.e., one that is lined with mesoderm). Except in leeches, the coelom is partially subdivided by septa. Hydrostatic pressure is maintained across segments and helps maintain body rigidity, allowing muscle contractions to bend the body without collapsing it.

The internal organs of annelids are well developed. They include a closed, segmentally-arranged circulatory system. The digestive system is a complete tube with mouth and anus. Gases are exchanged through the skin, or sometimes through specialized gills or modified parapodia. Each segment typically contains a pair of nephridia. The nervous system includes a pair of cephalic ganglia attached to double nerve cords that run the length of the animal along the ventral body wall, with ganglia and branches in each segment. Annelids have some combination of tactile organs, chemoreceptors, balance receptors, and photoreceptors; some forms have fairly well developed eyes, including lenses.

Annelids may be monoecious or dioecious. Larva may or may not be present; if present they are of the trochophore type. Some forms also reproduce asexually. They are protostomes, with spiral cleavage.

Members of the Phylum Annelida can be found throughout the world, in marine, freshwater, and terrestrial environments. Ecologically, they range from passive filter feeders to voracious and active predators.


Class Polychaeta

Polychaetes include such forms as sand worms, tube worms, and clam worms. Most have well developed, paired, paddle-like appendages (parapodia), well developed sense organs, and numerous setae (usually on the parapodia; "polychaete" means "many hairs"). Polychaetes usually have a well-developed head, often complete with well-developed eyes, antennae, and sensory palps. They lack any permanent sex organs (in contrast to other kinds of annelids); gonads appear as swellings during the breeding season. Gametes are shed into the coelom and carried outside the body through the nephridia or as a result of the body wall actually rupturing. Fertilization is external, and development proceeds indirectly through a trochophore larva. Polychaetes are a large and extremely diverse group. Around 10,000 species have been described. Most are marine. Some, such as featherduster worms, are sedentary, living in tubes buried in sand or mud, and feed by trapping food particles in mucus or by ciliary action. Others, such as the clam worm, are active, mobile predators that capture prey in jaws attached to their pharynges. Still others, such as fireworms, graze on gorgonians and stony corals.

Polychaetes are extremely abundant in some areas. They play essential ecological roles, serving on one hand as predators on small invertebrates, and on the other as food for fish and large invertebrates.


Class Oligochaeta

The oligochaetes include earthworms and a group of related, mostly freshwater, species of annelids. Over 3000 species are known. Oligochaetes all bear setae, but the number of setae present is much smaller than in polychaetes (their name means "few hairs"). Oligochaetes also differ from polychaetes in that they possess permanent sex organs. Most are hermaphroditic, and development is direct, resulting in young that resemble tiny adults. Aquatic forms may have gills. Oligochaetes feed primarily on detritus and algae. Earthworms cycle huge quantities of soil through their guts, a process that speeds the turnover of nutrients in soil and increases productivity. They also help to aerate soil. Aquatic oligochaetes are important food for fishes and larger invertebrates. A few are ectoparastic.


Class Hirudinea

Leeches are unlike other annelids in several ways. They have a fixed number of segments (usually 34), a dorso-ventrally flattened body, both an anterior and posterior sucker (usually), no parapodia, and usually no setae. The coelom is not subdivided by septa in most species, and it has been filled with muscle and connective tissue. Leeches are hermaphroditic. Development is direct as in oligochaetes.

Most leeches are found in freshwater habitats, but a few are marine and some are terrestrial (but they require warm, moist conditions). Most are either carnivorous or parasitic. Medicinal leeches were used for centuries by physicians to control diseases that were believed to be caused by an excess of blood. Interest in using leeches has revived recently among surgeons trying to reattach severed limbs or digits, for it turns out that leeches are able to do a better job of controlling swelling in the reattached limb (while minute veins grow and reconnect) than can surgeons. Scientists have also shown much interest in the anticoagulant that leeches secrete as they feed.

Approximately 500 species of leeches have been described.


Phylum Arthropoda

Arthropods include an incredibly diverse group of taxa such as insects, crustaceans, spiders, scorpions, and centipedes. There are far more species of arthropods than species in all other phyla combined, and the number of undescribed species in the largest assemblage of arthropods, the insects, probably numbers in the tens of millions. Members of the phylum have been responsible for the most devastating plagues and famines mankind has known. Yet other species of arthropods are essential for our existence, directly or indirectly providing us with food, clothing, medicines, and protection from harmful organisms. The systematic relationships of arthropod groups is not fully understood, which is not surprising given the size and diversity of the phylum. Here, we will follow the scheme recommended by Hickman and Roberts (1994), supplemented with information from Brusca and Brusca (1990), Pearse et al. (1987) and lectures by R. D. Alexander.

A number of important characteristics are shared by most members of this phylum. Arthropods are bilaterally symmetrical protostomes with strongly segmented bodies. Segmentation affects both external and internal structure. Some segments are fused to form specialized body regions called tagmata; these include the head, thorax and abdomen, and the process and condition of fusion is called tagmosis. The body is covered with an exoskeleton made up primarily of the protein chitin; lipids, other proteins, and calcium carbonate also play a role. Primitively, each body segment bears a pair of segmented (jointed) appendages; in all living arthropods, many of these appendages are dramatically modified or even lost. Arthropods generally grow by molting their exoskeletons in a process called ecdysis. Movement of appendages is controlled primarily by a complex muscular system, divided into smooth and striated components as in chordates. Cilia are not present. Most arthropods have a pair of compound eyes and one to several simple ("median") eyes or ocelli; either or both kinds of eyes may be reduced or absent in some groups. Arthropods are eucoelomate with the coelom formed by schizocoely, but the volume of the coelom is much reduced and usually restricted to portions of the reproductive and excretory systems. Most of the body cavity is an open "hemocoel," or space filled loosely with tissue, sinuses, and blood. The circulatory system is open and consists of a heart, arteries, and the open spaces of the hemocoel. The gut is complete. Respiration takes place through the body surface, and/or by means of gills, tracheae, or book lungs. The nervous system is annelid-like, with a brain (=cerebral ganglion) and a nerve ring surrounding the pharynx that connects the brain with a pair of ventral nerve cords. These cords contain numerous ganglia. Most arthropds are dioecious and have paired reproductive organs (ovaries, testes). Fertilization is internal in most but not all groups. Most lay eggs, and development often proceeds with some form of metamorphosis.


Subphylum Trilobita

Trilobite, common name for a class of extinct marine arthropods. Trilobites ranged in length from a few millimeters up to about 65 cm (26 in), although most species were between 3 and 7 cm (1 and 3 in) long. Trilobites lived during the Paleozoic era 570 to 250 million years ago and were most prevalent in the early part of that era. The trilobites were named for the arrangement of their exoskeleton, or outer shell, into three lobes. The exoskeleton, the part of the organism that is most commonly preserved, was made of a hard substance; it covered the back of the animal.

Trilobites had two compound eyes. In some trilobites, the eyes had densely packed lenses and may have served merely as a light sensitive warning device to detect movement. In other trilobites, the eyes had fewer and more complex lenses and may have been capable of forming images and perceiving depth.

Trilobites lived in shelf and slope environments around continental margins and in the shallow continental seas that covered areas of the earth that today are land masses. Most trilobites were bottom dwellers, although some may have been swimmers or floaters. Some that possessed exceptionally large eyes and a large field of vision, such as Carolinites, are thought to have been swimmers that inhabited surface waters. Others, with reduced eyes or no eyes at all, preferred deeper, darker waters. Many trilobites, such as Olenellus, burrowed into the sea bottom for protection and to seek food.

Trilobites employed a variety of feeding strategies. Many plowed through mud at the bottom of oceans and seas, ingesting the sediment to sort out organic matter. Others were scavengers or predators. Most trilobites could roll themselves up into a defensive position so that only the exoskeleton was exposed.

The fossilized remains of trilobites are useful because they help scientists develop relative time scales for the ancient marine environment. Because trilobites evolved quickly and were widely distributed, comparing the trilobite fossils found in rock layers in different regions of the earth can indicate which rock layer is older than the other. Trilobite fossils are particularly helpful in developing time scales for the early Paleozoic era.


Subphylum Chelicerata

These are the scorpions, spiders, mites, horseshoe crabs, and "sea spiders." Their bodies have two major subdivisions, an abdomen and a cephalothorax. Chelicerates have six pairs of appendages, which are uniramous (unbranched). These include a pair of chelicerae, a pair of pedipalps, and four pairs of walking legs. Chelicerates lack mandibles and antennae. Respiration is by means of book gills, book lungs, or tracheae.


Subphylum Crustacea

Approximately 30,000 species make up this Subphylum. Most are aquatic; of these, the majority are marine but some are found in fresh water. Members of the Subphylum include lobsters, crabs, crayfish, shrimp, copepods, barnacles, and several other groups of organisms. All have two pairs of antennae, a pair of mandibles, a pair of compound eyes (usually on stalks), and two pair of maxillae on their heads, followed by a pair of appendages on each body segment (crustacean bodies usually are made up of head, thorax, and abdomen, although the segments composing these tagmata differ among different Classes). The appendages are primitively branched (biramous), and although this condition is modified in many species, adults always have at least some biramous appendages. Crustaceans respire via gills. Like other arthropods, all have a hard but flexible exoskeleton. Most crustaceans are free-living, but some are sessile and a few are even parasitic. Most use their maxillae and mandibles to take in food. The walking legs, including specialized chelipeds, may be used to help capture prey. Some crustaceans filter tiny plankton or even bacteria from the water; others are active predators; while still others scavenge nutrients from detritus.

Most crustaceans are dioecious. The actual mechanisms by which fertilization is achieved vary greatly. Some crustaceans hatch young that are like miniature adults; others go through a larval stage called a nauplius.

Many species, including lobsters, crayfish, barnacles, and crabs are important to human economies, some very much so. Others, such as krill, are at the base of extremely important marine food chains. Still others are crucial in recycling nutrients trapped in the bodies of dead organisms.


Subphylum Uniramia

Uniramians are arthropods whose appendages are unbranched. Most appendages are made up of several articulating pieces. The uniramian body has two or three tagmata, and the abdomen contains numerous segments. The head appendages include paired antennae and mandibles, and also two pairs of maxillae (the second pair may be fused or sometimes absent). "Breathing" is by means of tracheae and spiracles. The sexes are separate, but most other aspects of reproduction are extremely varied. This enormous group include millipedes, centipedes, and insects, plus two small, primitive Classes, Pauropoda and Symphyla.


Class Chilopoda

Centipedes are uniramian arthropods whose bodies are made up of a chain of many (up to 177) flattened segments, each except the one behind the head and last two bearing a single pair of appendages (legs). The appendages of the first body segment have been modified to form large, poisonous fangs that are used to capture prey. The bite of a large centipede, however, can be painful to an adult and dangerous to a small child.

Centipedes are predatory, feeding on soil invertebrates such as earthworms and terrestrial insects. All centipedes are terrestrial, but they require moist microhabitats. Fertilization is internal, with spermatophore transferred in ways similar to many arachnids. Centipedes lay eggs, which in some species are carefully brooded by the female. When they hatch, the young resemble miniature adults.

Centipedes are a diverse group, including some 20 families and over 2500 species. Most are small, but a few attain up to 10 inches in length.


Class Diplopoda

Like centipedes, millipedes have bodies that are made up of numerous segments. The first four thoracic segments each bear a single pair of legs, but the following abdominal segments all have two pairs. Millipedes lack poisonous fangs and do not bite; rather, to discourage predators they roll into a defensive ball and many emit poisonous or foul-smelling substances.

Most of the approximately 8000 species of millipedes are herbivorous or scavengers, living primarily on decaying plant and animal matter in moist microhabitats. They are adept and powerful burrowers. Like centipedes, female millipedes lay eggs in nests, which are often carefully guarded. Newly hatched millipedes usually have only 3 pairs of legs, adding legs and body segments with each molt as they grow.


Class Insecta

With around one million named species and perhaps several times that number unnamed, insects account for a great majority of the species of animals on earth. They are a tremendously successful group. Insects can be found in almost all terrestrial and freshwater habitats, from the driest deserts to freshwater ponds, from the canopy of a tropical rainforest (where their diversity is unbelievably great) to the arctic wastes. A few species are even marine. Their feeding habits are similarly varied; almost any substance that has nutritive value is eaten by some group of insects. Insects also show huge variety in shape and form. Almost the only condition their group does not attain is very large body size. A number of features, however, are shared by most kinds of living insects. In addition to the general characteristics of uniramians, these include a body composed of three tagmata, a head, thorax, and abodmen; a pair of relatively large compound eyes and usually three ocelli located on the head; a pair of antennae, also on the head; mouthparts consisting of a labrum, a pair of mandibles, a pair of maxillae, a labium, and a tonguelike hypopharynx; two pairs of wings, derived from outgrowths of the body wall (unlike any vertebrate wings); and three pairs of walking legs.

Insects have a complete and complex digestive tract. Their mouthparts are especially variable, often complexly related to their feeding habits. Insects "breathe" through a tracheal system, with external openings called spiracles and increasingly finely branched tubules that carry gases right to the metabolizing tissues. Aquatic forms may exchange gases through the body wall or they may have various kinds of gills. Excretion of nitrogenous waste takes place through Malpighian tubules. The nervous system of insects is complex, including a number of ganglia and a ventral, double nerve cord. The ganglia are largely independent in their functioning; for example, an isolated thorax is capable of walking. Yet ganglia also use sensory output. A grasshopper with one wing removed can correct for this loss and maintain flight, using sensory input from its brain. Sense organs are complex and acute. In addition to ocelli and compound eyes, some insects are quite sensitive to sounds, and their chemoreceptive abilities are astounding.

Insects are dioecious and fertilization is internal in most. The ways in which mating is accomplished, however, are incredibly variable; study of this variability by evolutionary biologists has greatly advanced our understanding of the evolution of behavior, social evolution, and traits such as number, size of young and patterns of investment in them. Reproduction by insects often involves a male locating a receptive female through chemicals (pheromones) released by the female. In most species, females store the sperm in a special receptacle in their abdomens; even species that lay huge numbers of eggs (in honeybees, for example, the number may be over one million), females mate only once and rely on sperm stored during that mating for the rest of their lives.

The manner in which growth is accomplished is an especially important characteristic of insects. In some, hatching eggs produce miniature adults, which to grow must shed their exoskeleton in a process called ecdyisis. In almost 90% of insect species, however, newly hatched young are completely different in appearance from adults. These larval forms usually live in different habitats, eat different foods, and assume a body form completely different from that of their parents. The larva feeds and grows, molting its skin periodically. At some point larval growth is completed, the larva stops feeding and builds a case or cocoon around itself. In this nonfeeding condition it is called a pupa or chrysalis. While so encased, the larva undergoes a complete transformation or "metamorphosis" of its body form, and a fully-formed adult emerges. Insects that experience this sort of complete change are called "holometabolous." Other species undergo a more gradual process, in which the newly hatched young are more similar to the adult but are small in size, lack wings, are sexually immature, and may differ in other, relatively minor ways as well. The young in these insects are called nymphs, and the lifestyle is referred to as "hemimetabolous."

Insects are incalculably valuable to man. Usually, we think of them in a negative context. Insects eat our food, feed on our blood and skin, contaminate our dwellings, and transmit horrible diseases. But without them, we could not exist. They are a fundamental part of our ecosystem. A brief and incomplete list of their positive roles would include the pollination of many, perhaps most higher plants; the decomposition of organic materials, facilitating the recycling of carbon, nitrogen, and other essential nutrients; the control of populations of harmful invertebrate species (including other insects); the direct production of certain foods (honey, for example); and the manufacture of useful products such as silk and shellac.


Phylum Echinodermata

Phylum Echinodermata is the phylum most closely related to phylum Chordata, our own phylum. The 6,000 species that make up this phylum do not show body segmentation, and are radially symmetrical when fully grown for the most part. Almost all of the species are marine, although a few can live in brackish water. However, echinoderms are bilaterally symmetrical in the larvae stage, meaning that they are not closely related to animals of phylum Cnidaria, which never show bilateral symmetry.

This phylum is closely related to the chordates because the coelom of the animal is made from the digestive tube, not from cell masses like the phyla Mollusca, Annelida, and Arthropoda. Therefore, echinoderms are deuterosomes. Another relation to our phylum is an endoskeleton, made of 95% calcium carbonate. There are projecting spikes out of the endoskeleton, which is held together by skin tissue. Another hallmark of the echinoderms are hard, spiny skin. This is a common feature, but not always apparent in echinoderms. In fact, the word Echinodermata means "like a sea urchin's skin" in Greek. The uniting feature of echinoderms is a water-vascular system. This is a system of canals branching throughout the body that branch into many sections called tube feet. There are at least 2,000 tube feet, which can penetrate the body wall and skeleton in places called ambulacral grooves, in most echinoderms. These tube feet, and in many echinoderms arms and even organs, can be regenerated. There are five major classes in the phylum Echinodermata.


Class Crinoidea

The crinoids (sea lilies and feather stars ) are the most ancient and, in some respects the most primitive, of echinoderms. Attached stalked crinoids, called sea-lilies, flourished during the Palaeozoic* era, and some 80 species still exist today.

The crinoids tend to be the least familiar group of the echinoderms, since most tend to occur in deep water, although one species, ANTEDON BIFIDA, is quite common in shallow water around the British Isles.

Crinoids are suspension feeders. Antedon uses the tube feet to trap particles of food. The mucus secreted by the tube feet is passed to a ciliated groove lying along the axis of the pinnule, by the constant bending and flicking of the tube feet. Particles of food are trapped in the mucus and are passed down the pinnules and the arms to the mouth.

Although Antedon is not a particularly active animal, it is able to swim, somewhat inefficiently, by the slow beating movements of the arms.


Class Asteroidea

Class Asteroidea consists of the sea stars. They usually have five arms, but can have as many as 10-42 arms! A sea star has suction cup like tube feet which extend at ambulacral grooves through the skin and are used to pull the sea star along the floor. The mouth of a sea star is on the undersurface, and the anus is on the top. A sea star can eat a bivalve like an oyster by grabbing the shell with the tube feet and positioning its mouth in the opening between the hinged jaws. The stomach of the sea star comes out of its mouth and enters the oyster to digest the soft parts before returning to its original spot. Because the digestive tract and coelom of a sea star develop from the same part of the gastrula as in a lancelet, a chordate, sea stars are thought to be very closely related to our phylum.

Class Asteroidea is in the Phylum Echinodermata. The species that make up this phylum do not show body segmentation, and are radially symmetrical when fully grown but bilaterally symmetrical in the larvae stage. Almost all of the species are marine, although a few can live in brackish water. The coelom of the animals in this phylum is made from the digestive tube, not from cell masses. Therefore, echinoderms are deuterosomes. Echinoderms have an endoskeleton, made of 95% calcium carbonate. Another hallmark of the echinoderms is hard, spiny skin. This is a common feature, but not always apparent in echinoderms. The uniting feature of echinoderms is a water-vascular system. This is a system of canals branching throughout the body that branch into many sections called tube feet. There are at least 2,000 tube feet, which can penetrate the body wall and skeleton in places called ambulacral grooves, in most echinoderms. These tube feet, and in many echinoderms arms and even organs, can be regenerated. There are five major classes in the phylum Echinodermata.


Class Ophiuroidea

The brittlestars , or serpent stars, are so called for their long, slender, fragile arms, which are set off sharply from the circular, pentagonal, or slightly star-shaped body disk. The arms of brittlestars are flexible and appear jointed because of the conspicuous plates of the outer surface. They bear a row of spines along each edge. In one group, the basket stars, they are repeatedly branched, forming a large mass of tentaclelike limbs. Each arm contains a radial canal (or one of its branches), but it does not contain body organs. Brittlestars feed on detritus and small organisms. The mouth leads to a large saclike stomach that fills most of the body cavity. There is no intestine or anus, and solid waste is extruded through the mouth. The stomach is folded into ten pouches, between which lie ten respiratory sacs that open by slits onto the oral surface. The cells lining the sacs have flagellae, which create a current of water moving in and out. Respiratory exchange occurs chiefly through the thin lining of the sacs.


Class Echinoidea

Echinoids— sea urchins , heart urchins, and sand dollars —are echinoderms without arms and with a spiny shell, or test, formed of tightly fused skeletal plates. The sea urchins (regular echinoids) are hemispherical in shape, round on top and flat on the lower surface. They have very long, prominent spines and are often brightly colored. The test of a sea urchin is divided into ten parts from pole to pole, like the sections of an orange. Five of these are ambulacra, with openings for tube feet; these alternate with wider sections, called interambulacra, that lack tube feet. However, spines and pedicellaria are found over the entire surface of the test. Urchins move by pushing against the substratum with the spines and extending the tube feet in the direction of movement. If turned over they can right themselves by means of the tube feet on the aboral surface. The mouth, located in the center of the undersurface, is surrounded by a thickened region bearing five pairs of short, heavy tube feet and sometimes five pairs of bushy gills. Within the mouth is an elaborate five-sided jaw structure called Aristotle's lantern that can be partially extruded from the mouth. It is able to grind up calcareous exoskeletons of plants and animals. The anus is at the center of the aboral surface and is surrounded by a thin-walled area without skeletal plates.

Sand dollars and heart urchins (irregular echinoids) have a dense covering of short spines, and locomotion is exclusively by movement of the spines. There are two groups of podia-bearing ambulacra, one arranged in a petallike pattern on the upper surface and the other forming a similar pattern on the lower surface. The upper tube feet function as respiratory organs (there are no gills around the mouth), and the lower ones are specialized for gathering food particles. Sand dollars are extremely flattened and oval in outline; the anus is on the oral surface. Heart urchins are somewhat flattened and are heart-shaped; a deep ambulacral groove running from top to bottom creates a secondary bilateral symmetry. The anus is on the aboral surface, opposite the groove.


Class Holothuroidea

The sea cucumbers are long-bodied echinoderms with the mouth at or near one end and the anus at or near the other. Because of their elongation along the oral-aboral plane, they lie on their sides rather than on the oral surface. In nearly all sea cucumbers the skeleton is reduced to microscopic ossicles imbedded in the leathery skin. Sea cucumbers have no arms, but tube feet around the mouth have been modified to form a circle of 10 to 30 tentacles of varying lengths and shapes that function in gathering food particles from the ocean bottom. The gut of the sea cucumber terminates in a chamber called the cloaca that opens into the anus. Two unique structures called respiratory trees, found in most sea cucumbers, also terminate in the cloaca. These are systems of highly branched tubes, one on either side of the body. The animal pumps water into the respiratory trees by contracting the cloaca, and oxygen diffuses through from the walls of the trees into the fluid of the body cavity. The madreporite in most sea cucumbers opens into the body cavity rather than to the outside and receives its fluid from the cavity. In a few sea cucumber species there is a large mass of tubules at the base of the respiratory tree that can be shot out of the anus if the animal is irritated. The extruded tubules, which may engulf and incapacitate an intruder, break off; they are then regenerated by the sea cucumber. In other species the respiratory trees, gonads, and part of the digestive tract are shot out through the anus; this evisceration is followed by regeneration of the lost organs.


Phylum Chordata

Chordates are defined as organisms that possess a structure called a notochord, at least during some part of their development. The notochord is a rod that extends most of the length of the body when it is fully developed. Lying dorsal to the gut but ventral to the central nervous system, it stiffens the body and acts as support during locomotion. Other characteristics shared by chordates include the following (from Hickman and Roberts, 1994):


Subphylum Urochordata

The urochordates, more commonly called tunicates, are mostly sedentary animals that live on the sea bed, and obtain their food by passing a current of water through the body. Like most sedentary marine animals, the tunicates have planktonic larvae which float in the ocean currents and disperse the animals to new areas.

The larvae of the tunicates are small, tadpole-like animals, which have a strong, muscular tail to swim through the water. They clearly show the three chordate characters of a NOTOCHORD , DORSAL NERVE CORD and PHARYNGEAL CLEFTS .

These free swimming larvae have only a short life, and they then settle down on the sea bed. They undergo a remarkable metamorphosis in changing into the adult form. Note that the adult animals do not show any sign of a notochord, although they do have pharyngeal clefts and a dorsal nerve cord. The adults are so unlike the higher chordates that it is difficult to believe that they belong to the same phylum.

The best known examples of urochordates are the sea squirts which belong to the class Ascidiacea.


Subphylum Cephalochordata

The cephalochordates contain about fourteen species, the best known of which are the members of the genus Branchiostoma, which are commonly called amphioxus . They are found throughout the world, living on the sea bed among shell gravels. They live half buried in the sand, with the head projecting upwards, and feed by filtering water through their pharynx to extract small particles.

These animals look like an archetypal chordate - the type of animal from which higher forms evolved. But this is probably incorrect. The cephalochordates have some very unusual features, such as a unique excretory system and a very primitive nervous organisation. They probably separated from the main line of vertebrate evolution at a very early stage.


Subphylum Vertebrata

Almost the entire phylum Chordata is composed of the animals in the subphylum Vertebrata. There are two main hallmarks of this phylum: a skull and a backbone containing vertebrae. Vertebrae are a series of segmented units (exemplifying segmentation in this subphylum) that enclose the nerve cord. The skull encloses the brain, so together, it and the vertebrae enclose the major parts of the nervous system. The skull and vertebrae are part of the endoskeleton, another feature that all members of the subphylum Vertebrata show, but is not unique to the subphylum. The endoskeleton of a vertebrate is made of either hard bone or flexible cartilage, both made of primarily nonliving material secreted by living cells. There are seven classes within the subphylum vertebrata, listed below.


Class Myxini

The adjective which best describes the Myxini is "Lovecraftian". Hagfish are long, slender and pinkish, and are best known for the large quantities of sticky slime which they produce. Hagfish have three accessory hearts, no cerebrum or cerebellum, no jaws or stomach, and will "sneeze" when their nostrils clog with their own slime. They are found in cold ocean waters of both hemispheres, scavenging dead and dying fish but also preying on small invertebrates.

Hagfish are almost blind, but have well developed senses of touch and smell. They have four pairs of sensing tentacles arranged around their mouth. The mouth lacks jaws, but a hagfish is equipped with two pairs of tooth-like rasps on the top of a tongue-like projection. As this tongue is pulled back into the hagfish's mouth, the pairs of rasps pinch together. This bite is used to tear into the flesh of dead and dying fish which have sunk to the muddy ocean bottom, or in catching and eating marine invertebrates. By far, the largest part of their diet is polychaete worms, but because of their slow metabolism, hagfish may go for up to seven months without eating any food.

Unlike many other fish, the Myxini undergo direct development, with no larval stage. The newly hatched young are practically miniature versions of their parents. Young are hermaphroditic at first, bearing both sets of sex organs; later in life, they will be either male or female, but may change sex from season to season.

Hagfish eggs are approximately one inch long, and encased in a tough shell. These eggs are large for a fish, and a female can therefore not produce very many. Despite the low number of eggs laid, hagfish exist in large numbers, with populations of up to 15,000 occurring in a relatively small area. This suggests that hagfish have a low mortality rate.


Class chondrichthyes

Sharks, skates, rays, and even stranger fish make up the Chondrichthyes, or "cartilaginous fish." First appearing on Earth almost 450 million years ago, cartilaginous fish today include both fearsome predators and harmless mollusc-eaters (harmless, that is, unless you are a mollusc). A number of shark and ray species are fished, commercially or for sport. Members of the Chondrichthyes all lack true bone and have a skeleton made of cartilage (the flexible material you can feel in your nose and ears). Only their teeth, and sometimes their vertebrae, are calcified; this calcified cartilage has a different structure from that of true bone. Thus, preservation of the whole body of a cartilaginous fish only takes place under special conditions. This complete fossil rhinobatoid (guitarfish -- one of the earliest rays), Rhinobatis, shown on display at the Senckenberg Museum in Frankfurt, Germany, is from the Upper Cretaceous of Haqel, Lebanon, a place that has yielded many complete fossil sharks and rays.


Class Osteichthyes

The earliest remains of the bony fish are found from the Lower to Middle Devonian. These remains were already of well diversified forms, with the fleshy-finned and ray finned types already separate, and it is believed that the Osteichthyes must have arisen at some time during the Silurian* .

There are two subclasses; Actinopterygii and Sarcopterygii.

The distinguishing character concerns the structure of the paired fins. The fins of the SARCOPTERYGII (below right) are fleshy; in these fish the skeleton supporting the fin and its associated musculature project outside the body, thus forming a fleshy base. The fins of the ACTINOPTERYGII are formed by the dermal fin rays; the endoskeleton and muscles controlling the fins remain within the body.


Class Amphibia

There are three orders of living amphibians containing about 170 genera. The majority of amphibians are found in tropical regions with abundant rainfall. Correlated with the moisture requirement of amphibians is the fact that most forms are nocturnal.

The class name means "two lives", indicating that amphibians are able to survive both in water and on land. They represent an important step in the phylogeny of vertebrates, the bridge from water onto land. No amphibians live in the ocean, but some will tolerate brackish water.

Amphibian lungs are poorly efficient. For this reason, most amphibians also use their moist skin for respiration. Temperature is another key factor in the life of amphibians. Being ectothermic, their body temperature changes with the environment. Temperature extremes are deadly to amphibians. Toads will burrow below the frost line to escape winter temperatures and many frogs and salamanders will hibernate in the mud at the bottom of rivers and lakes. Some species actually produce glycerol, a type of "antifreeze", in their cells to resist ice crystal formation.

Hot, dry conditions are just as deadly. Many species of amphibian will escape these conditions by estivating, the summer equivalent of winter hibernation. They conserve body moisture by retreating into a moist burrow or crevice, curling tightly and remaining inactive, torpid, until better conditions return. Some even secrete a lipid, fatty substance, and coat their body to prevent water loss during this period of inactivity.


Order Urodela

The newts appear to be rather primitive amphibians because they have a rather simple shape.

Note that their legs are rather weak and small, and that the body often partly drags along the ground when the animal is moving. They rely on lateral undulations of the body to assist with movement. But these primitive features are probably deceptive, and there is no evidence to suggest that they arose any earlier than the other orders of modern amphibia.

Most newts and salamanders are found in North America, and there are smaller numbers of species in Europe and Asia. They are not found, for some unknown reason, in the southern hemisphere. Most species are only a few inches long, although the largest species - the Japanese and Chinese giant salamanders - can reach over 1 metre in length.

Many amphibia are fully aquatic and often have greatly reduced legs. All urodeles MOVE slowly on land, but in water they are often efficient and elegant swimmers.


Order Anura

The anurans have become highly specialised for a hopping method of locomotion. Most species can walk slowly, rather like newts, but they rely on HOPPING for fast movements. They are extremely efficient at this method of locomotion - the world record for a frog hop is over three metres.

The whole body skeleton has become very light, by the reduction and loss of bones. This is particularly apparent in the skull.

Another feature of the anurans is their feeding technique. Most frogs and toads feed on insects. They catch their prey by using a tongue which can be fired from the base of the mouth. The tongue is an organ which was first developed by the amphibians; no fish have such a structure. It can be rapidly unfolded and a sticky tip is used to pluck insects into the mouth.


Class Reptilia

Class Reptilia is made up of the first animals to produce amniotic eggs. In this type of egg, the embryo develops in a fluid filled sac called an amnion. This prevents the egg from drying out, and allowed the reptiles to live only on land. Reptiles have tough skin made of the protein keratin. Since reptiles do not need to breathe through the skin, it is much thicker than the skin of amphibians. Reptiles periodically shed their skin in a process called molting. Another adaptation allowing reptiles to live on land is a well-developed respiratory system with branched bronchial tubes in their lungs. Reptiles have teeth adapted for holding prey rather than chewing it because most species in this class swallow their prey whole. Reptiles have good hearing and vision and a tongue is used for smell as well as taste. The only disadvantage to reptiles is that they are ectothermic, meaning they warm up by absorbing heat rather than generating their own heat, like birds and mammals.

The Mesozoic Age is also known as the age of the reptiles because dinosaurs and other reptiles were the predominant animal during that period. Most of those species became extinct about 65 million years ago after a mass extinction possibly caused by a meteor or increased volcanic activity. However, one lineage of dinosaurs became the birds, and an earlier lineage led to the mammals, who also became more abundant after the age of reptiles. Other reptiles became the current orders, which include Chelonia, the turtles and tortoises, Crocodilia, the alligators and crocodiles, Squamata and Sauria, the snakes, and Rhynchocephalia, assorted other reptiles.


Order Squamata

The Order squamata, scaled reptiles, is the largest order of reptiles with over 6,000 living species. It is composed of three suborders: the Amphisbaenia (amphisbaenians), the Lacertilia (lizards), and the Serpentes (snakes). Members of this huge order are found worldwide, except in Antarctica and on a few very remote islands. All members of this order share similar characters in bone structures and anatomy of the male sex organ. All also have bodies covered in scales, and all periodically shed their skin. The squamata come in an amazing variety of sizes, shapes, and life styles. Scaled reptiles range in size from a 1.2 cm (0.5 in) long lizard to a 10 m (32.8 ft) long snake. Members of this order may be carnivorous or omnivorous. They live in a variety of habitats and may be aquatic, terrestrial, or arboreal. Fertilization is internal. Some species lay eggs; others bear live young.


Order Crocodilia

Crocodilians, along with birds, are the only survivors of the once-prevalent Archosauria. During the Mesozoic (245-65 million years ago) this group, including dinosaurs and other reptiles, dominated life on all continents and in the oceans. Most or all of crocodilians' adaptations had already evolved by the late Triassic (about 200 million years ago). Crocodilians are the most advanced surviving reptiles; many of their features are more similar to mammals or birds than to other reptiles.

There are only a few members, and all modern crocodilians have adapted to a semi-aquatic life, although as recently as 3000 years ago there may have been a terrestrial crocodilian species on New Caledonia. Some crocodiles may venture into larger bodies of fresh or salt water, but all must lay their eggs on dry land. Most crocodilians live in the tropics. The only exceptions are the American alligator (Alligator mississippiensis) and the Chinese alligator (Alligator sinensis), and they still cannot tolerate climates colder than temperate climates. No crocodilians venture out of lowlands; it is speculated that none ever lived above 1000 meters (3260 feet) above sea level.

All crocodilians have a similar body shape, with a head held horizontally in front of the body, four legs which project from the sides, heavy scales which function as armor, and a heavy muscular tail. Their front feet have five separate toes and their rear feet have four partially-webbed toes. Their eyes are on the top of their head, close together to allow for binocular vision (the field of vision of the two eyes intersects, to provide more accurate depth perception in front of the animal). The nostrils are crescent-shaped and valvular, and set at the end of the snout, which allows breathing even when the animal is almost entirely submerged.

Crocodilians range in size from Cuvier's dwarf caiman, which only grows to about 1.5 meters (five feet) long, to the Indopacific crocodile, which grows to 7 meters (23 feet) long. Male crocodilians are larger than females.

Their skin is covered with non-overlapping scales composed of the protein keratin and often studded with bony plates called scutes. (Lizard scales are similar, but lack the bony plates.) The scales are shed individually, so crocodilians do not molt (shed their skin all at once) like snakes do. This skin does not provide much insulation, preventing crocodilians from inhabiting cooler climates.

Crocodilians' normal gait is with their bodies off the ground; their ankles swivel to allow their legs to be almost underneath their body, making their gait resemble mammals'. No other reptiles move in this way. Crocodilians can also run by simply speeding up their walk; in smaller crocodilians this may change the gait into a "gallop" in which the animal appears to be bouncing; some may acheive speeds of 10 miles per hour (17 kph). Crocodilians can also move like lizards, moving one foot at a time with their bellies scraping on the ground; they also use this mode of movement when sliding down a river bank when frightened and sometimes occurs when they are running and their legs get out of sync.

Crocodiles swim with back-and-forth movements of their tail. Most of the time crocodilians cruise slowly through the water, holding their legs against their body to reduce drag. They are also capable of great bursts of speed, including a "tail walk" like dolphins do, in which their head and body are held vertically out of the water.

All crocodilians lay eggs in nests made out of plant material and/or mud; the nests may dry so hard that hatchlings would be trapped inside without help. Adults, especially mothers, often guard nests. All eggs in a nest hatch at the same time, and the entire brood leaves the nest at once. Adults will respond aggressively to hatchlings' distress cries, and mothers (and fathers, in some species) may attend hatchlings for several weeks. Crocodilians from the same brood stay together at least while they are young; in some species family groups stay together much longer.

Crocodilians' senses of smell, sight, and hearing are well developed. Their ears are covered by flaps which close to prevent water from entering them. Crocodilians' eyes are immobile spheres covered by three eyelids: the third eyelid, the nictitating membrane, is transparent, but protects the eye from water. They have vertical, cat-like pupils which dilate to allow them to see well in the dark. A layer of tapetum at the back of their eyes greatly increases their ability to see at night as well; this also makes their eyes glow in the dark. Crocodilians do not see well underwater.

Crocodilians have an elongate open-ear canal. They have two openings in their skull behind their eye sockets; it has been suggested that these openings help the attachment or functioning of crocodilians' powerful jaw muscles.

Their thoracic and abdominal cavities are separated by a muscular diaphragm, used in breathing. Their nostrils close when the animal dives; the nostrils are separated from the mouth by a bony palate (like mammals have, although other reptiles don't) and a valve in the back of the mouth. This allows crocodilians to breathe when their entire body except their nostrils are submerged, and also when holding prey.

Crocodilians' hearts have four chambers like mammals and birds, but there is a pore between the left and right ventricles which allows some mixing.

Crocodilians are poikilothermic (cold-blooded), and can only regulate their internal body temperature by arranging for their environment to warm them. Temperate species will bask in the sun during the day to raise their body temperature, returning to the water to cool off; they mostly hunt at night, leaving the daytime for basking in the sun. Opening their mouths can also cool them off, since the large exposed wet surface allows much evaporation. Tropical species may avoid the hot sun by remaining under water or mud during the day. Some crocodilians also estivate (sleep out the summer). Alligators, which live in temperate regions, may remain completely submerged except for their nostrils when the air is very cold in the winter. After feeding, crocodilians tend to seek more heat, as it speeds digestion.

Sexual maturity is reached once crocodilians reach a certain age and size (both are important: a crocodilian not big enough to become sexually mature may not, even if it is old enough). Crocodilians' sex is determined by the temperature of the environment at a critical stage in development; there is no X or Y chromosome, like birds and mammals have. Crocodilians continue to grow their entire lives, even after sexual maturity has been reached.

Crocodilians' teeth are mostly identical, thick-walled cones embedded in the outside of the jaw. Rear teeth are more cylindrical and blunter than front teeth. The teeth are not cemented into the jaw but simply attached to the outside, and can be broken off. Crocodilians can replace their teeth an indefinite number of times (as opposed to mammals, which only replace individual teeth once). Crocodilians do not have lips and their mouths leak when closed.

Their stomachs are the most acidic recorded for any vertibrate, allowing them to digest even the bones and shells of prey animals. Their digestion is also aided by a muscular gizzard containing stones to help break down food

All crocodilians have strong jaw muscles for biting and holding prey. They are all entirely carnivorous. Prey is not chewed or grinded in the mouth: once it is impaled on the sharp teeth, it is swallowed whole. Crocodilians need not feed often. A study on Nile crocodiles indicated that they usually eat only about 50 full meals a year. While their staple food is always fish, large crocodilians may eat mammals; smaller ones' diet may include insects, tadpoles, frogs, snails, crabs, shrimps, birds and small fishes. Snakes, molluscs, turtles and bats may also be eaten. Crocodilians are opportunistic hunters; they eat whatever they can catch if they are hungry. Flying prey can be caught by the crocodilians leaping into the air with thrusts of their powerful tail. Some larger crocodilians may also eat humans.

Food is stored as fat in the animals' tails, backs, and elsewhere in the body; up to 60% of the food intake may be converted to fat. This, the fact that they're poikilothermic (discussed above), and crocodilians' opportunistic hunting strategy allow crocodilians to survive for long periods with no food at all: large crocodilians may be able to survive up to two years between meals.

Crocodilians hunt by lying concealed until some prey comes close to them. They are capable of very short bursts of high speed to catch prey, but cannot maintain speed to give chase if the initial attack is unsuccessful. These extreme bursts of speed also produce large amounts of lactic acid, which crocodilians are inefficient at removing; after an attack, they must rest to clean out their blood and replenish their oxygen supply.

Crocodilians are normally solitary animals, but plentiful food may bring many individuals together. Some species have been observed to hunt cooperatively. Even when large numbers congregate, they do not seem to fight over food. When large prey is caught, another crocodile may help to dismember it, so that the peices are small enough to eat. Polygyny (males mate with more than one female) has been observed in all species studied.

Crocodilians communicate with each other by means of sounds, postures, motions, odors released by four scent glands, and by touch. Vocal sounds are made by forcing air through a voice box (larynx) in the throat. Young call to adults when in danger, but also are very vocal while being fed. Sounds (by the hatchlings themselves or by adults) also seem to keep young together. Adults also produce sounds to communicate with other adults. The most common adult sound is a loud, low roar which is repeated, and may be echoed by other adults. During mating, softer "purrs" are made. Threatened crocodilians may growl. Adults also grunt to signal to juveniles they will help, and may hiss while defending juveniles. They also communicate by slowly lifting their jaws off the ground with their mouth closed.

Adults are territorial, and mark their territory by loudly slapping their head down on the water or snapping their jaws on the surface of the water. Dominant animals tend to swim higher in the water; other crocodilians of the same species communicate their submission by swimming lower in the water. Dominant animals control access to mates, choice nesting sites, food, basking sites, and living space. During drought, territories are forgotten as crocodilians crowd into the smaller remaining inhabitable area, although hierarchies are still observed. In some species and in some areas, territories are only maintained by males or only during mating season.

Combat between crocodilians is rare, but does sometimes occur between animals of the same size competing for dominance. The two combatants line up next to each other facing opposite directions and bang the sides of their heads together; they also sometimes bite each other, but in either case they rarely cause any lasting damage.


Class Aves

Birds are vertebrates with feathers, modified for flight and for active metabolism. Birds are a monophyletic lineage, evolved once from a common ancestor, and all birds are related through that common origin. There are a few kinds of birds that don't fly, but their ancestors did, and these birds have secondarily lost the ability to fly. Modern birds have traits related to hot metabolism, and to flight:

There are about 30 orders of birds, about 180 families, and about 2,000 genera with 10,000 species. Most of them don't live in Michigan, though there are about 400 species that do.


Class Mammalia

All mammals share three characteristics not found in other animals: 3 middle ear bones; hair; and the production of milk by modified sweat glands called mammary glands.

Mammals hear sounds after they are transmitted from the outside world to their inner ears by a chain of three bones, the malleus, incus, and stapes. Two of these, the malleus and incus, are derived from bones involved in jaw articulation in most other vertebrates.

Mammals have hair. Adults of some species lose most of their hair, but hair is present at least during some phase of the ontogeny of all species. Mammalian hair, made of a protein called keratin, serves at least four functions. First, it slows the exchange of heat with the environment (insulation). Second, specialized hairs (whiskers or "vibrissae") have a sensory function, letting the owner know when it is in contact with an object in its external environment. These hairs are often richly innervated and well-supplied with muscles that control their position. Third, through their color and pattern, hairs affect the appearance of a mammal. They may serve to camouflage, to announce the presence of especially good defense systems (for example, the conspicuous color pattern of a skunk is a warning to predators), or to communicate social information (for example, threats, such as the erect hair on the back of a wolf; sex, such as the different colors of male and female capuchin monkeys; presence of danger, such as the white underside of the tail of a whitetailed deer). Fourth, hair provides some protection, either simply by providing an additional protective layer (against abrasion or sunburn, for example) or by taking on the form of dangerous spines that deter predators (porcupines, spiny rats, others).

Mammals feed their newborn young with milk, a substance rich in fats and protein that is produced by modified sweat glands called mammary glands. These glands, which take a variety of shapes, are usually located on the ventral surface of females along paths that run from the chest region to the groin. They vary in number from two (one right, one left, as in humans) to a dozen or more.

Other characteristics found in most mammals include highly differentiated teeth; teeth are replaced just once during an individual's life (this condition is called diphyodonty, and the first set is called "milk teeth); a lower jaw made up of a single bone, the dentary; four-chambered hearts, a secondary palate separating air and food passages in the mouth; a muscular diaphragm separating thoracic and abdominal cavities; highly developed brain; endothermy and homeothermy; separate sexes with the sex of an embryo being determined by the presence of a Y or 2 X chromosomes; and internal fertilization.

The Class Mammalia includes around 5000 species placed in 26 orders (systematists do not yet agree on the exact number or on how some orders are related to others). Mammals can be found in all continents and seas. In part because of their high metabolic rates (associated with homeothermy and endothermy), they often play an ecological role that seems disproportionately large compared to their numerical abundance.


Order Monotremata

Monotremes probably split from the lineage leading to other mammals sometime in the Mesozoic. They are often placed in a separate subclass from other mammals, Prototheria. They retain many characters of their therapsid ancestors (for example, a complex pectoral girdle, laying of eggs rather than bearing live young, limbs oriented with humerus and femur held lateral to body, and a cloaca). The skulls of monotremes are almost birdlike in appearance, with a long rostrum and smooth external appearance. Modern monotremes lack teeth as adults; sutures are hard to see; the rostrum is elongate, beak-like, and covered by a leathery sheath; and lacrimal bones are absent. Monotremes have several important mammalian characters, however, including fur (but they lack vibrissae), a four chambered heart, a single dentary bone, three middle ear bones, and the ability to lactate.

Besides the absence of teeth, lacrimals, and obvious sutures, monotremes share a number of skeletal characteristics. On the skulls, the jugals are reduced or absent, the dentary is a slender bone with only a vestige of a coronoid process, the angle of the dentary is not inflected medially (unlike that of marsupials), auditory bullae are missing (part of the middle ear is enclosed by tympanic rings), and much of the wall of the braincase is made up by the petrosal rather than the alisphenoid (unlike all other modern mammals). Postcranially, the skeleton of monotremes is also unique among mammals. It is a fascinating mosaic of primitive characteristics inherited from therapsids but found in no other living mammals, and modifications probably related to the burrowing habits of modern monetremes. Their shoulder girdles are complex, including the standard components of modern mammals (scapula and clavicle), but also additional elements including coracoid, epicoracoid, and interclavicle. The scapula, however, is simplified, lacking a supraspinous fossa. The shoulder girdle is much more rigidly attached to the axillary skeleton than in other mammals. Femur and humerus are held roughly parallel to the ground when the animal walks, more in the fashion of therapsids and most modern reptiles than like modern mammals. Ribs are found on the neck (cervical) vertebrae as well as the chest (thoracic) vertebrae; in all other modern mammals, they are restricted to the thoracic region.

Another interesting skeletal characteristic of monotremes is the large epipubic bones in the pelvic region. Epipubic bones were originally thought to be related to having a pouch, but they are found in both males and females. They also occur in all species of marsupials, whether a pouch is present or not (not all marsupials have a pouch). It is now thought that epipubic bones are a vestige of the skeleton of therapsids, providing members of that group with extra attachments for abdominal muscles to support the weight of the hindquarters.

Monotremes are endothermic, but they have unusually low metabolic rates and maintain a body temperature that is lower than that of most other mammals.

The eggs layed by monotremes are small (13-15 mm diameter) and covered by a leathery shell. The number of eggs laid is small, usually 1-3, and they are placed in the mother's pouch. They contain a large yolk, which is concentrated at one end of the egg very much like the yolk of a bird's egg. Only the left ovary is functional in the platypus, but both produce eggs in the echidna. Like the eggs of birds, monotreme eggs are incubated and hatched outside the body of the mother. Incubation lasts about 12 days. The young, which are tiny and at a very early stage of development when they hatch, break out of the eggs using a "milk tooth. They are protected in a temporary pouch in echidnas but not platypuses. They are fed milk produced by mammary glands; the milk is secreted onto the skin within the pouch and sucked or lapped up by the babies. Weaning takes place when the young are 16-20 weeks old.

All male monotremes have spurs on their ankles that are presumed to be used in fighting and in defense. In one family (Ornithorhynchidae), a groove along the spur carries poison secreted by adjacent glands.

Monotremes are restricted to Australia and New Guinea. Their fossil record is very poor; the earliest fossil attributed to this group is from the early Cretaceous. A fossil from Argentina suggests that the monotremes were more widely distributed early in their history.


Order Marsupialia

ANY OF AN ORDER (Marsupialia) of mammals comprising kangaroos, wombats, bandicoots, opossums, and related animals that do not develop a true placenta and that usually have a pouch on the abdomen of the female which covers the teats and serves to carry the young.


Class Insectivora

Almost all Mesozoic mammals were insectivorous. Insects are by far the largest group of organisms, both in terms of number of species and in biomass. They are marvelously diverse in habit and morphology. They provide a rich resource for anything that eats them, and their diversity alone suggests that their predators might exhibit similar variety. And so it is. Many mammals, including members of almost all orders, sometimes feed on insects. Members of one order, the Insectivora, feed almost entirely on invertebrates, especially insects. The Insectivora includes such groups as shrews, moles, hedgehogs, and tenrecs; these groups include around 375 species arranged in six living families. Tree shrews and elephant shrews have also sometimes been placed in this order; recent work, however, has shown the distinctiveness of these groups and they are now classified separately (orders Scandentia and Macroscelidea, respectively).

What traits characterize members of the Insectivora? Most insectivores are small; shrews, for example, are among the smallest mammals. Most rely more on their senses of hearing, smell, and touch than on vision. Some shrews can echolocate. The part of the brain that houses the sense of smell is especially well developed. The ear region of insectivores lacks an ossified bulla. The tympanic membrane is attached to a bony tympanic ring, and the middle ear may be partially enclosed by processes from adjacent bones. The jugal is reduced or may be absent, and the zygomatic arch is sometimes incomplete. The cheek teeth of many are dilambdodont, and even those with more derived molariform teeth tend to have cusps that can easily be identified according to the tribosphenic pattern. The incisors of some Insectivora are enlarged (but they are reduced in others), and the canines also vary considerably in morphology. The eyes are usually very small, the feet are plantigrade and have five digits, and neither the hallux or pollex is opposable.

Many of the traits used to define Insectivora are probably primitive for mammals. Whether the order is a natural (monophyletic) group is still open to question.

Members of the order are found through much of the world. They are missing from Australia and all but the northernmost part of South America. Most species eat invertebrates.


Order Chiroptera

Bats are unmistakable. No mammals other than bats have true wings and flight. Bat wings are modified forelimbs, much as are bird wings, except in the case of bats the flight surface is covered with skin and supported by four fingers, while in birds the flight surface is provided mostly by feathers and is supported by the wrist and two digits. The flight membrane usually extends down the sides of the body and attaches to the hind legs. Bats also often have a membrane called a uropatagium that runs between their hind legs and includes their tail (if they have one).

Bats are the second-most speciose group of mammals, after rodents. The approximately 925 species of living bats make up around 20% of all known living mammal species. In some tropical areas, there are more species of bats than of all other kinds of mammals combined.

Bats are found throughout the world in tropical and temperate habitats. They are missing only from polar regions and from some isolated islands.

There are two major groups of bats, usually given the rank of suborders, Megachiroptera and Microchiroptera The Megachiroptera includes one family (Pteropodidae) and about 166 species. All feed primarily on plant material, either fruit, nectar or pollen. The remaining 16 families (around 759 species) belong to the Microchiroptera. The majority of species are insectivorous, and insectivory is widely distributed through all microchiropteran families. But many microchiropterans have become specialized for other kinds of diets. Some are carnivorous (feeding on rodents, other bats, reptiles, birds, amphibians, even fish), many consume fruit, some are specialized for extracting nectar from flowers, and one group of three species feeds on nothing but the blood of other vertebrates. Megachiropterans and microchiropterans differ in many other ways. The "megabats" are found only in the Old World tropics, while "microbats" are much more broadly distributed. Microbats use highly sophisticated echolocation for orientation; megabats orient primarily using their eyes (members of one genus are capable of a primitive form of echolocation). Megabats control their body temperature with a tight range of temperatures and none hibernates; many microbats have labile body temperatures, and some hibernate. Megabats have claws on the second digits supporting their wings (with one exception); this is never the case in microbats. Megabats have relatively simple external ears; microbats often have large and relatively complex pinnae, including an enlarged tragus or antitragus. Microbats often have dilambdodont dentition or cheek teeth whose morphology can easily be related to dilambdodont teeth; megabats have simplified cheek teeth that are difficult to interpret. A controversy has arisen recently concerning whether the two major groups of bats are monophyletic. While this issue is not settled, at present most evidence seems to favor the hypothesis that they are monophyletic; that is, the most recent common ancestor shared by the two groups would also be classified as a bat.

An important cranial characteristic for recognizing bat families is the nature of the premaxilla.

The earliest fossil bat is a remarkably well preserved animal from early Eocene rocks in the Green River formation of Wyoming. Given the name Icaronycteris, it comes from a species that is clearly microchiropteran. This implies that the split between the two groups occurred substantially earlier, and the fossil sheds no light on the question of whether bats are monophyletic.


Order Primates

The Primates are an ancient and diverse eutherian group, with around 233 living species placed in 13 families. Most dwell in tropical forests. The smallest living primate is the pygmy mouse lemur, which weighs around 30 g. The largest is the gorilla, weighing up to around 175 kg.

Primates radiated in arboreal habitats, and many of the characteristics by which we recognize them today (shortened rostrum and forwardly directed orbits, associated with stereoscopic vision; relatively large braincase; opposable hallux and pollex; unfused and highly mobile radius and ulna in the forelimb and tibia and fibula in the hind) probably arose as adaptations for life in the trees or are primitive traits that were retained for the same reason. Several species, including our own, have left the trees for life on the ground; nevertheless, we retain many of these features.

Primates are usually recognized based on a suite of primitive characteristics of the skull, teeth, and limbs. Some of these are listed above, including the separate and well-developed radius and ulna in the forearm and tibia and fibula in the hindleg. Others include pentadactyl feet and presence of a clavicle. Additional characteristics (not necessarily unique to primates) include first toe with a nail, while other digits bear either nails or claws, and stomach simple in most forms (sacculated in some leaf-eating cercopithecids). Within primates, there is a tendency towards reduction of the olfactory region of the brain and expansion of the cerebrum (especially the cerebral cortex), correlated with an increasing reliance on sight and increasingly complex social behavior.

The teeth of primates vary considerably. The dental formula for the order is 0-2/1-2, 0-1/0-1, 2-4/2-4, 2-3/2-3 = 18-36. The incisors are especially variable. In some forms, most incisors have been lost, although all retain at least 1 lower incisor. In others, the incisors are intermediate in size and appear to function as pincers or nippers, as they commonly do in other groups of mammals. In some, including most strepsirhines (see next paragraph), the lower incisors form a toothcomb used in grooming and perhaps foraging. In the aye-aye (Daubentoniidae), the incisors are reduced to 1 in each jaw and are rodent-like in form and function. Canines are usually (but not always) present; they vary in size, including within species between males and females. Premolars are usually bicuspid (bilophodont), but sometimes canine-like or molar-like. Molars have 3-5 cusps, commonly 4. A hypocone was added early in primate history, and the paraconid was lost, leaving both upper and lower teeth with a basically quadrate pattern. Primitively, primate molars were brachydont and tuberculosectorial, but they have become bunodont and quadrate in a number of modern forms.

Living primates are divided into two great groups, the Strepsirhini and the Haplorhini. Strepsirhines have naked noses, lower incisors forming a toothcomb, and no plate separating orbit from temporal fossa. The second digit on the hind foot of many strepsirhines is modified to form a "toilet claw" used in grooming. Strepsirhines include mostly arboreal species with many primitive characteristics, but at the same time, some extreme specializations for particular modes of life. Haplorhines are the so-called "higher" primates, an anthropocentric designation if ever there was one. They have furry noses and a plate separating orbit from temporal fossa, and they lack a toothcomb. Haplorhines include many more species, are more widely distributed, and in most areas play a more important ecological role. Haplorhines are further divided into two major groups, the Platyrrhini and the Catarrhini. Platyrrhines have flat noses, outwardly directed nasal openings, 3 premolars in upper and lower jaws, anterior upper molars with 3 or 4 major cusps, and are found only in the New World (families Cebidae and Callitrichidae). Catarrhines have paired downwardly directed nasal openings, which are close together; usually 2 premolars in each jaw, anterior upper molars with 4 cusps, and are found only in the Old World (Cercopithecidae, Hylobatidae, Hominidae).

Most primate species live in the tropics or subtropics, although a few, most notably humans, also inhabit temperate regions. Except for a few terrestrial species, primates are arboreal. Some species eat leaves or fruit; others are insectivorous or carnivorous.


Order Lagomorpha

Currently, we recognize 80 living species of lagomorphs, placed in 2 families containing 13 genera. Native populations are found on all continents except Australia and Antarctica; they are absent from southern South America and most islands. Humans have introduced them, however, to many areas where they were originally not part of the fauna.

Lagomorphs are small to medium-sized animals that in many way resemble large rodents. They have a rudimentary or short tail. Folds of skin on the lips can meet behind the incisors so that gnawing can take place with the mouth cavity closed. Other flaps of skin are able to close the nostrils. The skull (especially the maxillary part of the rostrum) is peculiarly fenestrated. The palate is short. Lagomorphs have a pair of incisors in each quadrant of the upper jaw, one large and rodent-like, and the other a small peg located immediately behind the larger tooth. These teeth grow throughout the animal's life and have a layer of enamel that extends around to the posterior surface of the tooth (in contrast to rodent incisors, which have enamel on one face only. As in rodents, canines are absent and a large space (diastema) separates the incisors and the first cheek tooth. The cheek teeth are rootless and hypsodont. The crowns of the cheek teeth are relatively simple, with transverse basins separated by enamel ridges. The dental formula is 2/1, 0/0, 3/2, 2-3/3 = 26-28. The upper toothrows are more widely separated than the lower, so that occlusion can take place on only one side of the jaw at a time. As in rodents, the masseter provides most of the power for mastication; the temporalis muscle is relatively small. Another characteristic of lagomorphs is the location of the testes in males, which lie in front of the penis as in marsupials.

All lagomorphs are terrestrial. They occupy a wide diversity of habitats, ranging from tropical forest to arctic tundra. All are herbivores that feed on grasses and other small plants. Lagomorphs have the ability to produce two types of fecal material, one that is wet and eaten again for further nutrient absorption, and one that is dry and discarded.

The phylogenetic affinities of lagomorphs are controversial, although recent evidence has suggested that they may be related to rodents. But other groups have also been suggested as closest relatives, including marsupials, insectivores, primates, artiodactyls, and other ungulate groups!


Order Rodentia

With over 2000 living species placed in about 30 families, rodents are by far the largest order of mammals, at least in terms of number of taxa (well over 40% of mammalian species belong to the order Rodentia!). Rodents range in size from pygmy mice weighing 5 gms to capybaras, the largest of which weigh over 70 kg. They are found around the world except in Antarctica, New Zealand, and on some oceanic islands. Ecologically, they are incredibly diverse. Some species spend their entire lives above the ground in the canopy of rainforests; others seldom emerge from beneath the ground. Some species are highly aquatic, while others are equally specialized for life in deserts. Many are to some degree omnivorous; others are highly specialized, eating, for example, only a few species of invertebrates or fungi.

Despite their morphological and ecological diversity, all rodents share one characteristic: their dentition is highly specialized for gnawing. All rodents have a single pair of upper and a single pair of lower incisors, followed by a gap (diastema), followed by one or more molars or premolars. No rodent has more than one incisor in each quadrant, and no rodent has canines. Rodent incisors are rootless, growing continuously. Their anterior and lateral surfaces are covered with enamel, but their posterior surface is not. During gnawing, as the incisors grind against each other, they wear away the softer dentine, leaving the enamel edge as the blade of a chisel. This "self sharpening" system is very effective and is one of the keys to the enormous success of rodents.

The condition of a dominant pair of incisors used for gnawing, followed by a long diastema, is not unique to rodents, and in fact rodents are relative latecomers to this condition (even though as a group, they have a very old fossil history, going back to Paleocene times). It is even seen in a group of therapsids (ancestors of mammals), the tritylodonts, which lived during the Jurassic. Multituberculates, a very large and successful but now extinct group of early mammals, had a similar pattern. So do wombats, hyraxes, aye-ayes, and lagomorphs, to give a few examples chosen from modern mammals. Rodents have specialized in gnawing to an extreme, however, seen in few or no other groups of vertebrates.

The main muscle used in chewing by rodents is the masseter, and the rodents can be divided into several groups based on exactly how they use these muscles. These groupings have been used in several ways in the past to classify rodents.


Order Cetacea

The order Cetacea comprises two extant sub-orders and one extinct sub-order. The extant sub-orders are the Mysticeti (baleen whales) and the Odontoceti (toothed whales). The extinct sub-order is the Archaeoceti (ancient whales). Both the mysticetes and odontocetes are thought to be descendants of the archaeocetes. Cetaceans, along with bats, are considered some of the most derived mammals on the planet. They evolved from terrestrial animals to an entirely aquatic life form that is completely separated from the land in all aspects of biology. Cetaceans live, breed, rest, and carry out all of their life functions in the water. The phylogenetic relationships discussed above are in the currently accepted form but many biologists still classify the Mysticetes and Odontocetes as separate orders.

All cetaceans share a number of similarities: they have a fusiform, or streamlined body shape; paddle shaped front limbs; vestigial hind limbs (which are within the body wall); no external digits or claws; tail flattened laterally and bearing horizontal flukes at the tip; vestigial ear pinnae; basically hairless body (some young have hair on their snouts); thick subcutaneous blubber layer filled with fat and oil; telescoped skull bones; external nares (blowhole) on the top of the head; addition of compressed vertebrae; shortening of the neck; lack of sweat glands; internal reproductive organs; 3-chambered stomach; and an airway reinforced with cartilage down to the alveoli. Many of these characteristics are adaptations to reduce drag for fast swimming in an aquatic environment. Protuberances such as external ears or genitals would create turbulence and would be very inefficient for an animal in the water.

Many cetaceans can dive for extended periods of time to great depths. Some cetaceans are thought to be the most intelligent non-primates and many have proportionately large brains. They also have remarkably efficient lungs and circulatory systems.


Order Carnivora

The Carnivora is the result of a great radiation of mammals that ate meat. But not all meat eaters are in this order; carnivorous species can certainly be found among, for example, the marsupials, bats, primates, cetaceans, and others. It is also true that not all Carnivora are carnivorous; some, such as bears and raccoons, are decidedly omnivorous, and at least one, the panda, is primarily vegetarian. But the Carnivora are a clearly monophyletic group that is first known from the late Paleocene and whose primitive food habits were carnivorous.

Most members of the Order Carnivora can be recognized by their enlarged fourth upper premolar and first lower molar, which together form an efficient shear for cutting meat and tendon. These teeth are referred to as the carnassial pair. The exceptions are a few forms, such as bears, raccoons, and seals, in which these teeth are secondarily modified.

Carnivores tend to be medium-sized animals; too small and they couldn't find enough within their capacity to kill; too large and they wouldn't be able to satisfy their appetites. Most have very acute senses. Vision and hearing are excellent in many carnivores, and the sense of smell is often remarkable. Most have relatively large brains. Many are excellent runners. A few are good long-distance runners, but more commonly, Carnivora are rapid sprinters that use stealth to approach their prey, then overcome it with a short, violent rush. A few, like bears and raccoons, seem relatively slow or clumsy, but even these species are capable of remarkable bursts of speed. Even the long-distance runners don't have the highly modified and relatively inflexible skeletons and movement patterns of cursorial herbivores like artiodactyls; this is probably related to the often unpredictable demands that catching and killing large prey place on their skeletons.

The skulls of Carnivora are highly varied in form. Most have a well-defined, transverse glenoid fossa, and the dominant motion of the jaw is in the dorsal-ventral direction. The primary muscle powering the jaw is the temporal, and sagittal crest associated with the temporal is commonly a conspicuous part of the surface of the skull. Carnivores also have a strong zygomatic arch and a relatively large braincase. The auditory bullae and the turbinals also tend to be large and complex.

Besides usually having carnassials, almost all Carnivora retain the primitive number of incisors (3/3); an exception is the sea otter, which has 2/3. The outer (3rd) incisor is often relatively large and canine-like. The canines are large and conical. The number of teeth behind the carnassials varies considerably, from 1/1 in some cats to 4/4 in bears (would be more in some pinnipeds, but they have no carnassials). All teeth are rooted and diphyodont.

All Carnivora have a simple stomach.


Order Proboscidea

Elephants are the survivors of a radiation of giant herbivores that once were diverse and widely distributed, including as many as seven families and, through the Tertiary, many dozens of species. They are classified as "subungulates" and believed to be related to hyraxes and sirenians. Throughout their history, proboscideans have shown a tendency to increase in size. The earliest known members of the order were moderately large, probably weighing around 120 kg; modern species are very large (an adult male African elephant can weigh more than 6000 kg); and the largest elephant, an extinct species called Mammuthus trogontherii, was huge, weighing around 9000 kg, 1/3 again as much as African elephants. The earliest proboscideans lacked any trunk or tusks. These structures appeared in several different forms, with trunks of varying length and massiveness (inferred from the morphology of the skull; trunks are soft tissue that is not fossilized) and tusks in a variety of shapes, sometimes including lower as well as upper teeth.

Modern elephants have a long, muscular trunk that functions almost as a fifth limb, and males have a pair of huge tusks derived from upper incisors. Their cheek teeth are uniquely adapted to their highly abrasive diet. As was the case in their ancestors, modern elephants have 6 cheek teeth. The eruption pattern in modern elephants, however, is much modified. The first three cheek teeth are small and relatively simple. The fourth erupts and begins to function during the animal's fourth or fifth year. It moves forward gradually in the jaw, and is replaced after eight years by the fifth tooth. The sixth and last molar erupts when the elephant is about 25 years old. Only one molar is fully functional at a time. Each successive tooth is larger and more complex. The fourth through sixth molars are massive and include numerous transverse lophs or ridges, giving the tooth a "washboard" appearance. Elephants chew with a fore and aft motion of the jaw, grinding the food across the lophs.

The skull of modern elephants is short and high. The lamdoidal crest across the back of the skull is especially well developed; to it attach the muscles that bear the weight of the massive head, tusks, and trunk. The bone over the top of the head contains numerous air cells.

The skeleton of elephants is adapted to bear massive weight, and as well, to provide attachments for the muscles required by an animal that may feed by pushing over and uprooting entire trees. The limb bones are stout; neither ulna nor tibia is reduced in size, and the toes are splayed and supported at the heel by a pad of dense connective tissue. The orientation of the pelvis is also modified.

Elephants require huge amounts of food, over 400 kg per individual per day. They are capable of pushing down large trees in order to get at their foliage and bark. Foraging herds can be very destructive of cropland or forest.

Elephants live long lives (60-70 years). They have complex social system that involves groups of related invidividuals led by mature females. They have an excellent sense of smell but relatively poor sight and hearing. They communicate by means of a variety of sounds, some produced vocally and others by tapping or stamping on the ground. Elephants (especially Asian elephants) are economically extremely important in some parts of the world as beasts of burden.

There is a single family of elephants (Elephantidae) with two living species (but mammoths, which coexisted with and were hunted by early humans, belonged to this family). African elephants are found in Africa south of the Sahara; Asian elephants occur in India, Nepal, and Southeast Asia.


Order Sirenia

Four living species placed in two families make up this order. Sirenians, which are sometimes called sea cows, are large mammals that spend their entire lives in water. Their forelimbs are modified to form flippers, their hindlimbs are reduced to nothing more than a vestigial pelvis, and their tail is enlarged and flattended horizontally to form a fluke or paddle. Sirenians are massive, sometimes weighing over 1150 kg. Their body is streamlined and mostly nearly hairless. Their ears have no pinnae. Their eyes lack obvious eyelids, but are closed by a sphincter-like mechanism. Their bones are unusually dense, a condition called pachyostosis; the extra mass probably helps them remain suspended at or below the surface of the water. Their nostrils are located on top of their snouts and closed by valves. The lips are large and mobile, and they are covered with stiff bristles.

The skull of sirenians is unmistakable. The premaxillae are large and deflected downward. Nasals are reduced or absent, and the nasal opening extends posteriorly nearly to the orbits. The dentary is very broad. In the region of the ear, the tympanic bone is semicircular, and the petrosal is massive and only loosely bound to the basicranium. The teeth of manatees and dugongs are unusual, but they are very different from each other and are described in the accounts of each family.

Sirenians are members of the group known as subungulates, thought to be distantly related to hyraxes, elephants, and perhaps, artiodactyls and perissodactyls. Their fossil record goes back to the Eocene, but at that time both families were distinct and specialized for aquatic life, so their origin is likely to have been considerably earlier.

A fifth species of sirenian is extinct. The Stellar sea cow (genus Hydrodamalis) was a huge sirenian (probably over 6000 kg, the size of an African elephant!) related to dugongs. It lived in the Bering sea, where it fed on seaweed (no other mammal feeds exclusively on seaweed). Stellar sea cows were exterminated by sailors in the mid 1700's, shortly after their discovery. The remaining sirenians, manatees and dugongs, are seriously threatened by hunting, habitat degradation, and in the case of manatees, collisions with boats in the shallow coastal areas they prefer.

Sirenians are vegetarians, feeding on a variety of marine algae and higher plants. Members of both families are social, occuring in large aggregations and interacting frequently with one another.


Order Perissodactyla

The name Perissodactyla means "odd-toed." This group of ungulates includes horses, tapirs, and rhinos. The name of their order derives from the fact that their middle toe is larger than the others, and the plane of symmetry of the foot passes through it, a condition called mesaxonic. Most species have three digits on the hindfoot and three or four on the forefoot, but in some only a single digit, the third, remains. Some species have horns, but these are dermal structures without bony cores, and they are located on the nasals or frontals in the midline of the skull. This contrasts with the horns of artiodactyls, which have bone cores, are paired, and are located on the frontals. The anterior part of the skull of perissodactyls is elongated and accomodates a full series of large cheek teeth (most have a total of 44 teeth). Molars and premolars are hypsodont in grazing forms such as horses, and brachydont in browsers such as tapirs. Modern species are lophodont (complexly so in equids), in contrast to artiodactyls, which tend to be selenodont or bunodont. Perissodactyls have a simple stomach, in contrast to the chambered structure of most artiodactyls. Their cecum is enlarged and sacculate, and in it some bacterial digestion of cellulose takes place.

Early in the Tertiary, this was a dominant group that included 14 families and many species. One extinct species of rhinoceros, Indricotherium ( = Baluchitherium), was the largest land mammal that ever lived, standing approximately 5.4 m tall at the shoulder and weighing around 30,000 kg (5 times the weight of modern elephants!). Now, all that remain are 3 living families, with 18 species in all. Their decline accelerated during the Oligocene and coincided with the rise of another group of large herbivorous and cursorial mammals, the artiodactyls.

Modern perissodactyls are native to Africa, south and central Asia, southern North America, and northern South America. Most species are herbivorous.


Order Artiodactyla

The artiodactyls are a large and remarkably diverse group of mammals, containing around 220 living species placed in 10 families. The majority live in relatively open habitats, such as plains and savannas, but others dwell in forests, and one group is semiaquatic. Within the order can be found some of the fastest-running mammals, but the Artiodactyla also includes relatively slow and cumbersome species such as pigs and hippos.

Artiodactyls are paraxonic, that is, the plane of symmetry of each foot passes between the third and fourth digits. In all species the number of digits is reduced at least by the loss of the first digit, and the second and fifth digits are small in many. The third and fourth digits, however, remain large and bear weight in all artiodactyls. This pattern has earned them their name, Artiodactyla, which means "even-toed." Artiodactyls stand in contrast to the "odd-toed ungulates," the Perissodactyla, in which the plane of symmetry runs down the third toe. The most extreme toe reduction seen in any artiodactyls (living or extinct) is in forms such as antelope and deer, which have just two functional (weight-bearing) digits on each foot. In such forms the third and fourth metapodials fuse, partially or completely, to form a single bone called a cannon bone. In the hind limb of these species, the bones of the ankle are also reduced in number, and the astragalus becomes the main weight-bearing bone. These traits are probably adaptations for running fast and efficiently.

Artiodactyls tend to share a number of cranial and dental characteristics, but the group is so diverse that there are exceptions to each trait. The anterior part of the skull (in front of the orbits) tends to be long and narrow. Horns or antlers are often present, usually on the frontals, which are usually larger than the parietals. All artiodactyls have a postorbital bar or process. The number of teeth is variable, but in many species it is smaller than the number found in perissodactyls. A diastema usually separates anterior and posterior teeth, especially in the lower jaw. Cheek teeth are bunodont in some forms, but more commonly selenodont. The premolars tend to be relatively small and not fully molariform.

Most artiodactyls have modified stomachs, the extreme case being that of groups such as antelope and deer, which have distinctive, four-chambered stomachs. This arrangement appears to be an adaptation that allows members of these groups to make use of microorganisms to decompose cellulose into digestible components. Cellulose is an important constituent of plant tissue that most mammals cannot digest.

Artiodactyls are native to all continents except Australia and Antarctica. The group contains a number of domesticated species. The fossil record of artiodactyls is ancient, going back at least to the earliest Eocene. The group expanded greatly (up to 36 families have been described in the Cenozoic) and its members took on more-or-less modern form in the Miocene. The dominance of this order is correlated with the decline of perissodactyls. It has been suggested that artiodactyls caused most perissodactyls to disappear, or that the disappearance of perissodactyls permitted the radiation of artiodactyls, but we probably can never know whether either of these relationships occurred.