Organization of the Animal Body
Animals are characteristically multicellular heterotrophs whose cells lack cell walls. At some point during their lives, animals are capable of movement. In the most commonly encountered animals, this stage is the adult, although some animals (corals) have sessile (nonmobile) adult phases and mobile juvenile forms. Animal and plant evolutionary history both show the development of multicellularity and the move from water to land (as well as secondary adaptation back to water).
Animals developed external or internal skeletons to provide support, skin to prevent or lessen water loss, muscles that allowed them to move in search of food, brains and nervous systems for integration of stimuli, and internal digestive systems.
Most animals have a life cycle with a pre-adult stage, a predominance of the diploid stage, and a series of embryonic developmental stages.
Evolution and Classification of Animals
Animals probably evolved from marine protists, although no group of protists has been identified from an at-best sketchy fossil record for early animals. Cells in primitive animals show similarities to collared choanoflagellates as well as pseudopodia-producing
Multicellular animal fossils and burrows (presumably made by multicellular animals) first appear 700 million years ago, during the late precambrian time (the Proterozoic). All known Proterozoic animal fossils had soft body parts: no shells or hard (and hence preservable as fossils) parts.
The Cambrian: Animals with Hard Body Parts
Beginning 570 million years ago, the early Cambrian Period, animals with external skeletons appeared in abundance. This sudden appearance of fossils is in fact used to define the beginning of the Cambrian. These external skeletons were hard enough to be more readily preserved than the soft-bodied forms that had dominated early animal evolution.
Modern animals are classified into between 30 and 35 phyla: all major modern phyla were present at the beginning of the Cambrian, along with a great variety of now-extinct phyla recorded in the Burgess Shale. Of the animal phyla, scientists consider nine major invertebrate phyla and the chordates to be of major importance in terms of biological diversity. While all major animal phyla are represented by Cambrian fossils, reconstructing fossil history is extremely difficult since earlier, soft-bodied animals did not fossilize well. Consequently, evolutionary relationships have been established for the most part on a studies of living (extant) animal anatomy.
Trends in Animal Evolution
Most animals have a body plan best described as a "tube-within-a-tube". This plan calls for two openings: one for food to enter the body (mouth), one for wastes to leave the body (anus). Animals with the "tube-within-a-tube" plan are 10% more efficient at
digesting and absorbing their food than animals with the sac-like body plan. The tube-within-a-tube plan allows specialization of parts along the tube. The sac-like body plan has only one opening for both food intake and waste removal. Sac-like body plan animals do not have tissue specialization or development of organs.
Triploblasty: Three Tissue Layers
Many, but not all, animals have three tissue layers as they develop embryologically: the endoderm, mesoderm, and ectoderm. Some animals, most notably sponges, lack these tissue layers. Cnidarians (coral and jellyfish) have only two of these layers. Flatworms, ribbon worms, etc. all have three tissue layers. Humans are triploblastic.
Asymmetry and Symmetry
Asymmetrical animals (sponges) have no general body plan or axis of symmetry that divides the body into mirror-image halves. Radially symmetrical animals (such as coral and jelly fish) have body parts organized about a central axis and tend to be cylindrical in shape. Radially symmetrical animals often are (for some part of their life) nonmotile (sessile), and live attached to a substrate. Radial symmetry allows animals to reach out in all directions from one center. Bilaterally symmetrical animals (such as humans and fish) have only a single plane of symmetry that produces mirror halves. Bilaterally symmetrical animals tend to be active and to move forward at an anterior end, which eventually led to concentration of sensory organs in the anterior end, or head.
Different body plans. There are no planes of symmetry in the asymmetrical form, while the radial form has numerous planes. Bilateral organisms, like humans, have only one plane of symmetry, extending from the head to the toe. Bilateral symmetry is an important step in development of a head and concentration of sensory organs in that head.
Body Cavity and Development
Acoelomate animals (like flatworms and flukes) do not have a coelom (or body cavity) produced during preadult development.
Pseudocoelomate animals (like roundworms) have a body cavity but it does not develop from splitting of the mesoderm.
Coelomate animals (humans, fish, shrimp) have a body cavity lined with mesoderm cells.
Some animals have their bodies divided into segments. This allows them to specialize certain segments, such as for antennae, eyes, claws, etc. Humans, insects, and earthworms are examples of segmented animals.
Examples of body segments as seen in a crayfish. Segmentation of the body allows development of various specialized limbs, such as antennae, pincers,
walking legs, swimming legs, and feeding appendages.
Sponges: The Phylum Porifera
The Phylum Porifera ("pore-bearing") contains approximate 5,000 species of sponges. These asymmetrical animals have sac-like bodies that lack tissues, and are usually interpreted as representing the cellular level of evolution. Cells from fragmented sponges can reorganize / regenerate the sponge organism, something not possible with animals that have tissues. Sponges may have evolved from a colonial protozoan. There are no true tissues in sponges: merely specialized cell layers. Epidermal cells in sponges line the outer surface. Collar cells line the inner cavity. Beating collar cells produce water currents that flow through pores in sponge wall into a central cavity and out through an osculum, the upper opening. A 10 cm tall sponge will filter as much as 100 liters of water a day. Amoeboid cells occupy the "inner" layer, along with hardened structures known as spicules. Sponges feed by drawing water into the body through a network of pores (hence the name porifera, pore-bearer) and passing it out through the large opening (osculum) at one end of the body. Most zoologists consider sponges as offshoots that represent an evolutionary dead-end. Sponges are aquatic, largely marine animals, with a great diversity in size, shape, and color.
Sponges can reproduce asexually (by budding or from fragments) or sexually. Sponges produce eggs and sperm that are released into a central cavity of the sponge, in which the zygote develops into a ciliated larva. The larval stage is able to move about while the adult is stationary.
The fossil record of sponges has been at times quite good. The oldest sponges date from the precambrian. One early example of fossil sponges are the archaeocyathids, one of the first reef-building animals. Archaeocyathids evolved and went extinct before then end of the Cambrian Period. Cladistic analysis by J. Reitner in 1990 suggests archaeocyathids are properly placed in the Phylum Porifera instead of having their own phylum. Living sponges fall into three groups: the calcareous, glass, and demosponges.
Tissues: Jellyfish, Corals and Sea Anemones
The Phylum Cnidaria contains 10,000 species characterized by adult bodies having radial symmetry. Cnidarians are aquatic, mostly all marine. Cnidarians have only the ectoderm and endoderm tissue layers. Members of this phylum are characterized by stinging cells, that eject a barbed thread and possibly a toxin. Only cnidaria have cnidocytes, a specialized cell that contains a nematocyst, a fluid-filled capsule containing a long, spirally coiled hollow thread. When the trigger of the cnidocyte is touched, the nematocyst is discharged. Some threads merely trap a prey or predator, while others have spines that penetrate and inject paralyzing toxins spines that penetrate and inject paralyzing toxins.
Two body forms occur: a mobile medusa and a sessile (not mobile) polyp. Both forms have tentacles arranged around an opening into the two-layered sac-like body. The inner tissue layer (derived from endoderm) secretes digestive juices into the gastrovascular cavity, which digests food and circulates nutrients. Muscle fibers occur at the base of the epidermal and gastrodermal cells. Nerve cells located below epidermis near the mesoglea interconnect and form a nerve net throughout the body. Cnidarians have both muscle fibers and nerve fibers, making these animals capable of directional movement. The nerve net allows transmission of messages in more than one direction, possibly an advantage in a radially symmetrical animal, while contraction of muscle fibers (under control of the nerve fibers) allows for movement.
Cells are organized into tissues. The adult in most species is radially symmetrical. The typical cnidarian life cycle involves both sexual and asexual reproduction. A bilaterally symmetrical larva known as a planula, develops from a zygote. The planula settles
in an appropriate location and grown into the adult polyp, that reproduces asexually to form medusae. Each medusa develops gonads and forms gametes.
Life cycle of a typical cnidarian. In essence we see an "alternation of generations" between the sessile polyp phase and the mobile medusa phase. However, unlike plants, both phases are diploid.
Cnidarians are divided into a number of taxonomic classes.
Class Anthozoa: Sea Anemones and Corals
Sea anemones are solitary polyps 5-100 mm in height and 5-200 mm in diameter or larger. They are often brightly colored and look like flowers on the seafloor. Their thick, heavy body rests on a pedal disk and supports an upward-turned mouth surrounded by hollow tentacles. Sea anemones feed on various invertebrates and fish. They attach to a variety of substrates, or may be mutualistic with hermit crabs, living attached to crab's shell.
Corals may be solitary but most are colonial. Most are found in shallow waters; the accumulation of their calcium-carbonate remains builds reefs. Modern scleractinian coral, dominant reef-builders since the Triassic Period, have symbiotic photosynthetic dinoflagellates living within the coral body. These dinoflagellates are in the genus Symbiodinium, and are termed zooxanthellae.
Class Hydrozoa: Hydras and Portuguese Man-of-War (Physalia)
The polyp stage is dominant in members of this class. The Portuguese man-of-war is a colony of polyps, with the original polyp becoming a gas-filled float. Other polyps specialize for feeding or reproduction. The Portuguese man-of-war can cause serious
injury to swimmers since each tentacle has numerous nematocysts.
Hydra are solitary, freshwater hydrozoan polyps. The body is a small tube about one-quarter inch in length, best observed with a dissecting microscope. Four to six tentacles surround the mouth, the only opening at one end. Hydra can move from one location by gliding or even somersaulting. Hydras have both muscular and nerve fibers, and respond to touch. Epidermal cells are termed epitheliomuscular cells and contain muscle fibers. Cnidocytes and sensory cells are also present in the epidermis. Interstitial cells can produce an ovary or testis, and may assist regeneration. Gland cells secrete digestive juices into the gastrovascular cavity into which tentacles have stuffed captured prey. Digestion is completed within food vacuoles of nutritive-muscular cells. Nutrients diffuse to the rest of the body. Hydras reproduce both sexually asexually (by budding). In sexual reproduction, sperm from a testis swim to an egg within an ovary. Following early development within the ovary, a protective shell forms and allows the embryo to survive until conditions are optimum. Hydra are commonly utilized animals in biology labs.
Class Schyphozoa: True Jellyfishes (Aurelia)
The medusal stage is dominant in jellyfish; the polyp remains small. Jellyfishes also serve as food for larger marine animals. Image of the jellyfish Aurelia. Photo illustrates
the four circular gonads of the medusa phase, and the ring of tentacles at the rim of the medusa.
The Fossil record of Cnidarians
The fossil record of cnidarians is very good for corals, but usually not as good for soft-bodied forms like jellyfish. Corals become dominant reef-building animals during the Ordovician, and continue their importance today.
Bilateral Symmetry and Cephalization: Phylum Platyhelminthes
The Phylum Platyhelminthes contains about 13,000 species of flatworms subdivided into three classes: two parasitic and one free-living. The planaria and relatives are freshwater animals placed in the Class Turbellaria. Flukes are external or internal parasites belonging to the Class Trematoda. Tapeworms are internal parasites and form the Class Cestoda. The phylum as a whole has adult bilateral symmetry and cephalization (the development of a head with sensory organs, in most members).
Flatworms have three tissue layers: ectoderm, mesoderm and endoderm and a body plan that is acoelomate and sac-like with a single opening. The mesoderm layer gives rise to muscles and reproductive organs. Free living forms have muscles, a nerve cord, and digestive organs, but lack both the respiratory and circulatory systems common to the "higher" animals. Flatworms have a branched gastrovascular cavity that is the site of extracellular digestion and which distributes nutrients throughout the body. Gas exchange occurs by diffusion through the skin. Platyhelminthes have an excretory system that also functions as an osmotic-regulating system. Flatworms have ladder-style nervous system composed of paired ganglia that form a brain connected via nerve cells to sensory cells in the body wall. Parasitic members of this phylum, such as flukes and tapeworms, have characteristic modifications:
1.loss of cephalization producing a head bearing hooks and suckers to attach to the host 2.extensive development of the reproductive system with loss of other systems (what do they do but reproduce?)
3.lack of a well-developed nervous and gastrovascular system (the live in a fairly stable environment and the host has already digested their food)
4.development of a tegument that protects them from host digestive juices
Both flukes and tapeworms use secondary or intermediate hosts to transport the species from primary host to primary host. The primary host is infected with the sexually mature adult while the secondary host contains the larval stage(s).
The Class Turbellaria includes freshwater planaria such as Dugesia that feed on small organisms or the remains of small creatures. Their small size and ease of care also make them common animals in biology labs. The planarian head is arrow-shaped, with side extensions that are sensory organs for detection of food and the presence of other organisms. Flatworms have two light-sensitive eyespots that have pigmentation making them look cross-eyed. The presence of three muscle layers facilitates varied movement. Gland cells secrete a mucous material upon which the animal slides or glides.
The animal captures food by wrapping itself around prey, entangling it in slime, and pinning it down. The pharynx is a muscular tube that extrudes from the mouth and through which food is ingested.
The flame cell system functions in excretion and consists of a series of interconnecting canals that run length of the body on either side of the longitudinal axis and side branches of the canals, each ending in a flame cell. The flame cell is a bulb-shaped cell containing a tuft of cilia within the hollow interior of the bulb. The cilia move back and forth, bringing water into the canals that empty through pores at the body surface. This flame-cell system functions in both water excretion and osmotic regulation in a typical freshwater, free-living flatworm.
Planaria can reproduce both sexually and asexually. They can constrict beneath the pharynx and each half will grow into a whole animal by the process of regeneration. Planaria are hermaphroditic, possessing both male and female sex organs, and can cross-fertilize each other. Fertilized eggs are enclosed in a cocoon and hatch in two to three weeks.
The Class Trematoda includes flukes. Flukes, such as blood, liver, and lung flukes are named after the organs they inhabit. Fluke bodies tend to be oval and elongate. They lack a definite head but have an oral sucker surrounded by sensory papillae. Flukes have reduced digestive, nervous, and excretory systems. Reproductive systems are well developed and usually hermaphroditic. (Image is a Fasciola hepatica, a liver fluke, in a section of liver) The blood fluke causes schistosomiasis, a disease found predominantly in tropical Africa and South America. Unlike most flukes, blood flukes are male or female. Flukes deposit eggs in blood vessels around the host's intestine. The eggs migrate to the intestine and are passed out with feces. Larvae hatch in water and swim about until they detect and enter a particular species of snail. The larvae reproduce asexually and eventually leave the snail. Once larvae penetrate human skin they begin to mature in the liver, and implant in blood vessels of the small intestine. A weakened person is then more likely to die from secondary diseases.
The Chinese liver fluke requires two intermediate hosts. Humans become infected when they eat uncooked fish. Adults migrate to the liver and deposit eggs in the bile duct, which carries the eggs to the intestine. The larval flukes must then pass through two
intermediate hosts, a snail and a fish.
The Class Cestoda consists of the tapeworms. The tapeworm scolex has hooks and suckers that allow the organism to attach to the host's intestinal wall. Behind the head is a short neck and then a long string of proglottids. Each proglottid segment contains a
full set of both male and female sex organs and very little other structure. Since the animal does produce wastes, it retains its excretory canals, but no digestive system is needed. Being sessile organisms in a fairly stable environment, tapeworms have only
rudiments of nerves. Following fertilization, proglottids become a bag of eggs that when mature, breaks off and passes out with feces. If the eggs of tapeworms are ingested by pigs or cattle, the larvae become encysted in the muscle of the hosts. The covering of ingested eggs is digested away and the larvae burrow through the intestinal wall and travel by bloodstream to lodge and encyst in muscle; a cyst is a hard-walled structure sheltering a larval worm. If humans eat the meat of infected pigs or cattle and fail to cook it properly, they too become infected.
The Phylum Nemertea: Ribbon Worms
The Phylum Nemertea include approximately 650 species of marine ribbon worms. Ribbon worms have a distinctive reversible proboscis stored in a rhynchocoel; when the walls of the rhynchocoel are contracted, the proboscis is extended out of the body. The proboscis is a long, hollow tube that can be everted and shot outward through a pore located just above the mouth. It is used primarily for prey capture, and for defense, locomotion, and burrowing. This phylum is included as the organisms are also triploblastic. Several of the fossils from the Cambrian-aged Burgess Shale are interpreted as ribbon worms, and some extremely long worms have been found in the Mediterranean (up to 100 feet long) and in the ocean under the Antarctic ice shelves.
The Phylum Rotifera
Rotifers belong to the phylum Rotifera, which contains about 2,000 species. Rotifers are often observed in biology lab preparations. Their digestive tract is the inner tube and the rest of the animal is the outer tube. Rotifers are microscopic and abundant in freshwater. A crown of cilia (corona) forms a rotating wheel that serves as both an organ of locomotion and acts to deliver food to the mouth.
The Tube-within-a-tube Body Plan: Phylum Nematoda
The Phylum Nematoda consists of several hundred thousand species of roundworms. Most are free-living, although some are parasitic (pinworms are thought to infect 30% of all US children). Adult nematodes have a pseudocoelom (tube-within-a-tube), a closed fluid-filled space that acts as a hydrostatic skeleton, aids in circulation and dispersal of nutrients. Nematodes lack a circulatory system, but do have a well developed digestive system. Nematodes, like rotifers, are pseudocolelomates.
One nematode, Caenorhabditis elegans, has only 1000 genes in its genome and its developmental pathways are well known. C. elegans serves as a model for eukaryote gene systems and has been extensively studied as part of the human genome project.
Ascaris is a parasitic roundworm. These worms are unsegmented and have a smooth outside wall. They move by whiplike motions. Mating produces eggs that mature in the soil, limiting most are to warmer climates. When eggs are swallowed, larvae burrow through the intestinal wall, moving to the liver, heart and/or lungs. Once within the lungs, larvae molt and, after 10 days, migrate up the windpipe to throat where they are swallowed. In the intestine, the mature worms mate and the female deposits eggs that are lost with the feces. Feces must reach the mouth of the next host to complete the life cycle, thus, proper sanitation is an important aspect to prevent infection.
Humans contract Trichinella (which causes the disease trichinosis) by eating raw pork containing encysted larvae. Mature female adults burrow into the wall of the small intestine. Live offspring are carried by the bloodstream to the skeletal muscles where they encyst. Religious dietary injunctions against eating pork may in part be a reflection of the prevalence of this disease in the Middle East.
Filarial worms cause various diseases. Dirofilaria causes heartworm in dogs, and is a common filarial worm in temperate zones.
Elephantiasis, a disease associated with tropical Africa, and is also caused by a filarial worm that utilizes mosquitoes as secondary hosts. Adult worms reside in and block lymphatic vessels. This results in limbs of an infected individual swelling to monstrous size. Elephantiasis is treatable in its early stages but not after scar tissue has blocked lymphatic vessels.
The True Body Cavity (Coelom): Phylum Annelida
Although they show superficial similarities to nematodes, annelids are segmented and possess a true coelom. Segmentation is an important evolutionary advantage annelids share with humans. Segmentation refers to the segmentation of body units that contain similar structures. The advantage of segmentation is that it permits greater specialization. The coelom is a fluid filled space located between the body wall and the gut. It develops within the embryonic mesoderm and has a definite lining of epithelial cells, called the peritoneum. The peritoneum lines the inner surface of the body wall and forms the mesenteries that suspend the internal organs in their proper position. The development of a coelom was probably associated with an increase in size. With a body cavity, there is more space to accommodate coiled or looped internal organs. Extra space also allowed the lungs to expand and contract, the heart to beat, and the stomach to churn. The more than 12, 000 species of annelids are divided into three major classes: Oligochaeta, Polychaeta, and Hirudinae.
Members of this class include the common earthworm. The name itself means "few bristles". Earthworms are of tremendous importance to agriculture. They burrow in the soil increasing aeration and drainage. They also help break down organic matter to make nutrients available for crop plants. A combination of circular muscles, which surround the body, and longitudinal muscles which run along he length of the body, allows individual segments to contract. These contractions move along the body in a wave, a process called peristalsis. There are 4 pairs of chitinous bristles on each segment called setae. These can be extended to anchor the worm in its burrow and to assist in locomotion. Soil containing dead and decaying plant and animal material enters the mouth and is sent backwards to the crop. Here it is stored until it moves into the grinding organ called the gizzard. In the intestine digested food is absorbed into the blood via an extensive capillary network. Soil particles and undigested materials are expelled through the anus. Another feature of annelids not seen in lower groups is a closed circulatory system. There is no heart but muscular blood vessels in the anterior region act as pumps to help move the blood. The pigment hemoglobin is used to carry oxygen. The annelids organs of excretion are called nephridia. There is a pair in each of the approximately 100 segments. Partway along the body is a prominent swollen segment called the clitellum, which contains glands. When the eggs are laid, the clitellum secretes a cocoon which envelopes and protects them until they leave it as fully formed worms.
Polychaetes are a diverse group of marine worms. Some are transparent and free swimming, a few are parasitic, living within the coelom of other polychaetes. They have flattened fleshy projections called parapodia on either side of each segment that function in movement and gas exchange. They are armed with bristles which provide protection and traction.
This group is also known as the leeches. They are found in lakes, slow-moving streams, ponds and marshes, and on moist vegetation in jungles. About 75% are blood sucking external parasites. The leech attaches to the host and cuts through the skin to start a flow of blood. The leech's salivary glands produce a chemical called hirudin which prevents the host's blood from clotting.
Coelomates: Animals with Internal Body Cavities
Coelomates are animals that have internal body cavities, or coeloms. Humans are coelomates, we have an abdominal cavity (digestive organs, some of the excretory and reproductive organs) and a thoracic cavity (heart and lungs). Coelomates also form a variety of internal and external skeletons. External skeletons and coeloms appeared during the Cambrian-Ordovician time. These skeletons offered several advantages to their producers:
1. Secretion of a mineral shell that allowed the animal to use the shell as a mineral repository.
2. Protection from drying out in the intertidal zone during low tides.
3. Protection from predators.
4. Sites for anchoring muscle attachments, offering new patterns of locomotion and increased strength.
Phylum Mollusca: Clams, Scallops, and Squids
The phylum Mollusca contains over 100,000 species with a variety of body forms and lifestyles. In mollusks, the coelom is reduced and limited to the region around the heart. The Mollusk body first appeared during the Cambrian Period.
All mollusks have:
1. a visceral mass containing internal organs, including the digestive tract, paired kidneys, and reproductive organs.
2. a mantle that surrounds but does not cover entirely the visceral mass and secretes a shell (if one is present). The mantle also contributes to formation of gills or lungs.
3. a head/foot region containing sensory organs and a muscular structure (foot) used for locomotion. The foot is a muscular structure used for locomotion, attachment to a substrate, food capture, or a combination of functions. A radula is an organ that bears many rows of teeth and is used for grazing on food. The nervous system consists of several ganglia connected by nerve cords. Most mollusks have an open circulatory system: a heart that pumps hemolymph through vessels into a hemocoel. Blood diffuses back into the heart and is pumped out to the body again. Some mollusks are slow moving, and have with no head, while others are active predators that have a head and sense organs.
Classification of the Mollusca
The Class Polyplacophora
Chitons are in the class Polyplacophora. They have a shell that consists of eight overlapping plates. A ventral muscular foot is used for creeping along or clinging to rocks. Chitons feed by scraping algae and other plant food from rocks with a well-developed radula.
The Class Gastropoda
The class Gastropoda includes snails, terrestrial slugs, whelks, conchs, periwinkles, sea hares, and sea slugs. Most gastropods are marine, although some are freshwater or terrestrial. Many are herbivores that use their radula to scrape food from surfaces. Carnivorous gastropods use their radula to bore through a surface, such as a bivalve shell, to obtain food. Most gastropods have a well developed head with eyes and tentacles projecting from a coiled shell that protects the visceral mass. The coiled shells of gastropods are often quite commonly found as fossils. One genus, Turetella, occurs in such quantities in a type of rock that the rock is known as "Turetella agate". However, not all gastropods have shells, the nudibranchs (sea slugs) and terrestrial slugs lack shells.
In aquatic gastropods, gills are found in the mantle cavity; in terrestrial gastropods, the mantle is richly supplied with blood vessels and functions as a lung when air is moved in and out through respiratory pores. Terrestrial gastropod embryonic development does not go through a swimming larval stage, as is the case in aquatic gastropods. For terrestrial snails, their shell not only offers protection but also prevents desiccation (drying out). The muscular foot contracts in peristaltic waves from anterior to posterior causing secretion of a lubricating mucus.
Terrestrial gastropods are hermaphroditic. In premating behavior, they meet and shoot calcareous darts into each other's body wall. Each inserts a penis into the vagina for the other to provide sperm for future fertilization of eggs. Eggs are deposited in the soil and development proceeds without formation of a larval stage. Hermaphroditism assures that any two animals that meet can mate, especially useful in slow-moving animals.
The Class Bivalvia
The class Bivalvia consists of the bivalves: clams, oysters, mussels, and scallops. "Bivalves" are two-part shells that are hinged and closed by powerful muscles. The presence of shells in this group has yielded an impressive fossil record. The bivalves have no head, no radula, and little cephalization. Clams use their hatchet-shaped foot for burrowing; mussels use it to produce threads to attach to objects. Scallops can both burrow or swim. A rapid closing and opening of their two valves releases water in spurts. The bivalve shell is secreted by the mantle. The shell is composed of protein and calcium carbonate with an inner layer of pearl. Pearls form as layers of shell-forming material deposited about a foreign particle lodged between the mantle and the shell. A compressed muscular foot projects down from shell. By expanding the tip, the foot pulls the body after it. Beating cilia of the gills cause water to enter the mantle cavity by way of the incurrent siphon and to exit by way of the excurrent siphon. While cilia of gills move water through the mantle cavity, gills also capture particles in water and move them toward the mouth. From the mouth food goes to the stomach, then to the intestine, which passes through the heart and ends at the anus.
Bivalves, like other mollusks, have an open circulatory system. Their nervous system consists of three pairs of ganglia. Two excretory kidneys below the heart remove ammonia waste from the pericardial cavity into the mantle cavity, from which it will leave the body.
Sexes in class Bivalvia are separate. The gonad is located around the coils of the intestine. Certain clams and annelids have the same type of larva, hinting at a possible evolutionary relationship between the two groups.
The Class Cephalopoda
The class Cephalopoda (literally "head-footed") includes squids, cuttlefish, octopuses, and nautiluses (and extinct relatives, the goniatites, ammonoids, and ammonites). The presence of a shell in many representatives of this class has yielded an impressive fossil record.
Squids and octopuses can squeeze water from their mantle cavity out through a funnel, thus propelling them with a form of jet propulsion. Surrounding their head are tentacles with suckers that can grasp prey and deliver it to a powerful beak/mouth. Cephalopods in general have well-developed sense organs, including focusing camera-type eyes. Most cephalopods, especially octopuses, have well-developed brains and show a capacity for learning. Nautiluses are enclosed in shells, squids have a shell that is reduced and internal, while octopuses lack a shell.
Squids and octopuses possess ink sacs from which they squirt a cloud of ink, as a means of escaping predators. Squids possess a vestigial skeleton under the mantle, called the pen, which surrounds the visceral mass. A squid has three hearts, one pumps blood to internal organs; two pump blood to the gills in the mantle cavity. Gonads make up a large portion of the visceral mass. Cephalopds have separate sexes. Spermatophores contain sperm, which the male passes to the female mantle cavity by way of one of his tentacles. After fertilization, eggs are attached to the substratum in strings containing up to 100 eggs.
Phylum Arthropoda: Segmented Bodies with Segmented Appendages
The phylum Arthropoda contains animals with segmented appendages on their body segments. Arthropods occupy every habitat, and are in many respects the most successful animal group on Earth. There are conservatively over 1 million species of living arthropods. Biologist E.O. Wilson estimates there are 10 million species, 9 million of which are arthropods. Certain groups of arthropods have extremely complete fossil records. Arthropod features that have contributed to their success include:
1. A hard exoskeleton, a strong but flexible outer covering composed primarily of the carbohydrate chitin. This functions in protection, attachment for muscles, locomotion, and prevention of desiccation.
2. Presence of jointed appendages. Trilobites, which flourished during Cambrian Period and were important animals in marine ecosystems for the remainder of the Paleozoic Era, had a pair of appendages on each body segment. Modern arthropod appendages are specialized for walking, swimming, reproduction, etc. These modifications account for much of the diversity and success of arthropods.
3. A complex nervous system with a brain connected to a ventral solid nerve cord. The head bears various sensory organs. Compound eyes have many complete visual units, each of which collects light independently. The lens of each visual unit focuses the image on light sensitive membranes of a small number of photoreceptors within that unit. In simple eyes (like our own), a single lens brings the image to focus into many receptors, each of which receives only a portion of the image.
4. A unique respiratory system that employs a variety of respiratory organs. Marine arthropods utilize gills composed of a vascularized, thin-walled tissue specialized for gas exchange. Terrestrial forms have book lungs (e.g., spiders) or tracheae. (e.g., insects). Book lungs are invaginations to serve in gas exchange between air and blood. Tracheae are air tubes that serve as ways to deliver oxygen directly to cells.
5. A complex, yet adaptable, life cycle. Metamorphosis is a drastic change in form and physiology that occurs as an immature stage becomes an adult. Metamorphosis contributes to the success of arthropods because the larval stage eats food and lives in environments different from the adult; reducing competition between immature and adults of a species. Reduction in competition thus allows more members of the species to exist at one time.
The arthropod body consists of three major collections or zones of body segments:
Classification of Arthropods
Due to their great diversity of appendages, lifestyles, and other features, arthropods are usually separated into several subphylums.
The Subphylum Chelicerata
The subphylum Chelicerata includes spiders, scorpions, ticks, mites, horseshoe crabs, etc. The first pair of appendages are chelicerae, second pair are pedipalps, and the next four pairs are walking legs. Chelicerae are appendages that function as feeding organs. Pedipalps are feeding or sensory in function; although in scorpions, they are large pincers. All appendages attach to a cephalothorax, a fusion of the head and thoracic regions. The head lacks antennae, mandibles, or maxillae appendages.
The Class Merostomata
The class Merostomata contains the extinct "sea scorpions" (or eurypterids) and the extant (living) horseshoe crabs. Eurypterids are extinct, but were important elements of faunas 200-500 million years ago during the Paleozoic Era. Some were huge, reaching a length of over 10 feet. Some eurypterids may have been amphibious, emerging onto land for at least part of their life. Horseshoe crabs are an ancient group consisting today of only 5 species. Members of this class have a large shield that covers the cephalothorax. The compound eyes are reduced. The second pair of appendages, the pedipalps, resemble walking legs. They have a long, spike-like appendage called a telson that projects from the rear of their bodies. Respiration is via book gills (precursors to book lungs?).
The horseshoe crab genus Limulus is a familiar sight along the east coast of North America. The anterior shield is a horseshoe-shaped carapace with two compound eyes. The long, unsegmented telson projects to the rear. They possess book gills that resemble the pages in a book. Limulus is considered a living fossil due to its great similarity to fossil forms from the Paleozoic Era.
The Class Arachnida
The class Arachnida includes over 60,000 described species (and most likely a very large number of as yet undescribed ones) of spiders (around 35,000 species), mites and ticks (25,000 species), scorpions (1200 species), and other forms. Nearly all arachnids are terrestrial. Arachnids have a cephalothorax covered with a carapace-like shield. The abdomen may be segmented or unsegmented. Appendages on the abdomen are absent or modified, for example forming the spinnerets of spiders. Respiration is via tracheae or book lungs. Scorpions are arachnids. They are the oldest terrestrial arthropods known from fossils. All scorpions are nocturnal and spend most of the day hidden under a log or rock. Their pedipalps are large pincerlike appendages, and their abdomen ends in a stinger containing venom. Ticks are parasites that suck blood and sometimes transmit diseases. Chiggers are larvae of certain mites and feed on the skin of vertebrates.
Spiders have a narrow waist separating the cephalothorax from the abdomen. Spiders have numerous simple eyes rather than compound eyes. The chelicerae are modified as fangs with ducts from poison glands. The abdomen has silk glands used to spin a web to trap prey. Invaginations of the body wall form lamellae (pages) of the book lungs; air flows across the lamellae in the opposite direction from blood flow to exchange gases more efficiently.
The Subphylum Crustacea
The Subphylum Crustacea contains 30,000 mostly marine species. A few species live in freshwater. Lobsters, crabs, crayfish, shrimp, copepods, barnacles, and several other groups of organisms belong to this subphylum. All crustaceans possess 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 have a head, thorax, and abdomen. Crustaceans utilize gills for gas exchange. Most crustaceans are free-living, but some are sessile and a few are even parasitic. Some crustaceans filter tiny plankton or bacteria from the water, while others are active predators. A few crustaceans scavenge nutrients from detritus. Many species, including lobsters, crayfish, barnacles, and crabs are economically important (yum, yum). Krill, and a few other species, form the base of extremely important marine food chains. Still others are crucial in recycling nutrients trapped in the bodies of dead organisms. The subphylum contains several taxonomic classes. We will focus on one, the Class Malacostraca, which includes the shrimp, lobsters, etc.
The Class Malacostraca
The class Malacostraca is the largest taxonomic class of Crustaceans, having over 20,000 primarily marine species. Some malacostracans are freshwater, while others occupy diverse terrestrial habitats. Typical malacostracans include sowbugs, krill, and a very large order, the Decapoda, that contains many kinds of shrimp, crabs, and crayfish. Malacostracans typically possess a body with eight thoracic and six abdominal body segments, each bearing a pair of appendages. Class Malacostraca contains a number of economically significant species, such as edible lobsters, shrimp, crayfish and crabs. Many malacostracans contribute to plankton and as such are at the base of an immensely important marine food chain.
The Subphylum Uniramia
This subphylum contains arthropods that have unbranched appendages. The uniramian body has two or three tagmata, and an abdomen that has many segments. Appendages in the head region include paired antennae and mandibles, and also two pairs of maxillae. Gas exchange is by means of tracheae and spiracles. This subphylum include millipedes, centipedes, and insects.
The Class Chilopoda
This taxonomic class includes 20 families and more than 2500 species of centipedes, all terrestrial. Most centipedes are small, but a few can attain a length of up to 10 inches (25 cm). Centipedes have bodies are made up of a chain of many (up to 177) flattened segments. With the exception of the segment behind the head and the last body segment, each segment has 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.
The Class Diplopoda
Millipedes comprise this class containing some 8000 species. Bodies of members of this class are made up of numerous segments. Millipedes lack poisonous fangs and do not bite. Predators are discouraged by the millipede's rolling into a defensive ball. Production of poisonous or foul-smelling substances also serve to dissuade any would be predators. Most millipedes are apt burrowing herbivores or scavengers.
The Class Insecta
Insects are the largest group, with probably over one million identified and named species (and undoubtedly a greater number as yet unknown to us). Insects live in almost all terrestrial and freshwater habitats, with a few species living in the oceans.
Many insects have some thoracic appendages modified for flight. Insects are important as pollinators for flowering plants, as well as for the damage they do annually to crops, and the diseases they transmit (malaria, some forms of encephalitis, Dengue Fever, etc.).
(Image of a Dewy Dragonfly at left and cockroach below)
Insects display a wide huge variation in body styles, although there seems to be a size limit on the insect-style of body organization. Common features shared by most living insects include:
* body composed of three tagmata
* one pair of fairly large compound eyes
* usually three ocelli located on the head
* one pair of antennae on the head
* three pairs of walking legs
* two pairs of wings
* mouthparts consisting of a labrum, a pair of mandibles, a pair of maxillae, a labium, and a tonguelike hypopharynx
Insects have a complete, complex digestive system. They exchange gases through a tracheal system, with external openings called spiracles dividing into finely branched tubules that carry gases directly to metabolizing tissues. Aquatic forms may exchange gases through the body wall or may have various kinds of gills. Excretion of nitrogenous waste takes place via Malpighian tubules. The nervous system of insects is complex, including a number of ganglia and a ventral, double nerve cord. 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 excellent. Growth patterns are quite variable. Some insects hatch from eggs as miniature adults, which in turn shed their exoskeleton. Most insect species have newly hatched young that are completely different in appearance from adults. These larval forms usually live in different habitats, eat different foods, and look completely different from their adult stages. When larval growth is completed, the larva stops feeding and builds a case or cocoon around itself. In this nonfeeding condition (pupa or chrysalis) the larva undergoes a complete transformation or "metamorphosis" of its body form, eventually emerging as a fully-formed adult.
Insects are very valuable to us. While insects eat our food, feed on our blood and skin, contaminate our dwellings, and transmit diseases, we could not exist if they were not here. Insects are a vital part of our ecosystem, functioning in:
* pollination of many flowering plants
* decomposition of organic materials
* recycling of carbon, nitrogen, and other essential nutrients
* control of populations of harmful invertebrate species (including other insects)
* direct production of certain foods like honey
* manufacture of useful products such as silk and shellac
So, have you hugged a bug today?
There are 6000 species of echinoderms; all extant species are marine. The phylum Echinodermata includes the sea urchins, sea stars, sea cucumbers, and starfish. Most adults have radial symmetry, while their larvae are bilaterally symmetrical. Echinoderms have an endoskeleton consisting of calcareous plates bearing spines. Radial symmetry appears to be an advantage to the mostly bottom-dwelling echinoderms, who can thus feed in every direction. Adult echinoderms have no brain. Members of the phylum have a water vascular system that powers their multitude of tube feet. There are several taxonomic classes of echinoderms of varying familiarity to the general public.
The Class Crinoidea
The class Crinoidea includes about 600 species of crinoids, the stalked sea "lilies" and the motile feather stars. Their branched arms are used for filter-feeding and give the animals a flowerlike or plantlike appearance (hence the term sea lily). Crinoid stalks and blastoid heads are common fossils in certain parts of North America.
The Class Holothuroidea
The class Holothuroidea has 1,500 species of sea cucumbers. Sorry, these are NOT good eating as a general rule! Holothuroideans have a long leathery body, and feed by tentacles located around their mouth.
The Class Echinoidea
Scientists generally place about 950 species of sea urchins and sand dollars are in the class Echinoidea. Both sea urchins and sand dollars have spines that they use for locomotion, defense, and burrowing. Sea urchins have long, blunt spines.
Sand dollars are flattened with a five-part flowerlike pattern of pores for skin projections.
The Class Ophiuroidea
The class Ophiuroidea consists of approximately 2,000 species of brittle stars. Brittle stars have a central disk from which long, flexible arms radiate. These long arms allow them to move rapidly.
The Class Asteroidea
The class Asteroidea contains about 1,500 species of sea stars (commonly known as starfishes). Most starfish have a dorsoventrally flattened body. Starfish have a central disk to which five, or a multiple of five, sturdy arms are attached. Sea stars are common along rocky coasts where they eat clams, oysters, and other bivalves.
The five-rayed body has an oral (mouth) and aboral (upper) side. Spines project from the endoskeletal plate through the thin dermis. Pincerlike pedicellarie keep the surface free from particles. Gas exchange is conducted by skin gills. On the oral surface, each arm has a groove lined with tube feet.
A sea star feeds by everting its stomach. It positions itself over a bivalve and attaches tube feet to each side of the shell. By working tube feet in an alternating fashion, it opens the shell open. Only a small crack is needed to insert its cardiac stomach into the prey. Stomach enzymes begin digesting the bivalve as it is trying to close its shell. Partially digested food is then taken into the pyloric stomach for complete digestion. A short intestine opens at the anus on the aboral side.
Each arm has a well-developed coelom containing a pair of digestive glands and male or female gonads. The nervous system is a central ring with radial nerves in each arm. A light-sensitive eyespot is at the end of each arm, facilitating coordinated but slow responses. Locomotion depends upon the water vascular system. Water enters on the aboral side through the sieve plate, which is also known as the madreporite. Water passes through a stone canal to a ring canal and into the radial canals in each arm. The radial canals feed into lateral canals extending into tube feet, each of which has an ampulla. Contraction of an ampulla forces water into the tube foot, expanding it; when the foot touches a surface, the center withdraws forming a suction and adhering to surfaces. Echinoderms lack complex respiratory, excretory, and circulatory systems. Fluids within the coelomic cavity carry out the function of diffusing substances and gases. Gas exchange occurs across the skin gills and tube feet. Nitrogenous wastes diffuse through coelomic fluid and across the body wall. Cilia on the peritoneum lining the coelom keep the coelomic fluid moving. Sea stars reproduce both sexually and asexually. If the body is fragmented, each fragment can regenerate a whole animal. Sea stars spawn and release either eggs or sperm at the same time. The bilateral larvae undergoes a metamorphosis to become a radially symmetrical adult.
The Phylum Hemichordata includes about 90 species of acorn worms. Some acorn worms living on tidal mud flats have a proboscis, a collar, and a trunk. The dorsal nerve cord in the collar and trunk resembles the nerve cord of chordates. The pharynx below the collar has gill slits. Larva of hemichordates resembles the larva of echinoderms. These facts cause some scientists to believe echinoderms and hemichordates share a common ancestor and that hemichordates and chordates are related by a common ancestor.
Hemichordates are classified into three classes, two with living members, one consisting of only fossil forms.
The Phylum Chordata includes about 45,000 species that occupy nearly all environments. All chordates at sometime during their life history have:
* a notochord: a dorsal supporting rod located dorsally just below the nerve cord; it provides support and is replaced by the vertebral column in vertebrates
* a dorsal hollow nerve cord: a fluid-filled canal; spinal cord is protected by vertebrae
* pharyngeal gill pouches: openings that function in feeding, gas exchange, or both
These features are seen only during embryonic development in most vertebrates. Not all chordates are vertebrates. In the invertebrate chordates, fish, and amphibian larvae, pharyngeal gill pouches become functioning gills. Terrestrial vertebrates have their pouches modified for various purposes; in humans, the first pouches become the auditory tubes, the second become tonsils, and the third and fourth pairs become the thymus and parathyroid glands. Most chordates have an internal skeleton against which muscles work. Most have a postanal tail that extends beyond the anus; in some (like humans), this may only appear in embryos. The evolutionary origin of chordates remains a mystery, although biochemistry and comparative embryology indicates echinoderms and chordates share a common ancestry.
Not all chordates are vertebrates. Some chordates are invertebrates, lacking a vertebral column. In these invertebrate chordates, the notochord persists and is never replaced by the vertebral column.
The subphylum Urochordata contains 1,250 species of unicates that have gill slits. Adults have a body composed of an outer tunic with an incurrent and excurrent siphon. When they are disturbed, tunicates tend to squirt water out. Water passes into a pharynx and out numerous gill slits, the only chordate characteristic that remains in adults. Microscopic particles adhere to a mucous secretion in the pharynx and are eaten. The larvae are bilaterally symmetrical and have the three chordate characteristics. Tunicate larva metamorphose into the sessile adult. Beating of numerous cilia lining the inside of the pharynx creates a current to move water through a tunicate. Some suggest larvae became sexually mature without developing tunicate characteristics; thus, a urochordate larva was ancestral to vertebrates; or a cephalochordate larva may have been ancestral to vertebrates.
The 23 species of lancelets have three chordate characteristics and are in the subphylum Cephalochordata. Their body resembles the lancelet, a two-edged surgical knife. They inhabit shallow coastal waters; they lie partly buried in sandy substrates and filter feed. Lancelets feed on microscopic particles filtered from the constant stream of water that enters the mouth and exits through the gill slits into an atrium. Lancelets retain the three chordate characteristics as an adult. The notochord extends from head to tail, accounting for the name "Cephalochordata." Lancelets have segmented muscles.
The vertebrates comprise a large group of chordates, and are subdivided into seven classes (3 classes of fish, amphibians, reptiles, birds, and mammals). Vertebrates have an internal skeleton of cartilage or bone, with vertebrae surrounding the dorsal nerve cord.
The subphylum Vertebrata consists of about 43,700 species of animals with backbones. Vertebrates exhibit all three of the chordate characteristics at some point during their lives. The embryonic notochord is replaced by a vertebral column in the adult. The vertebral column is made of individual hard segments (vertebrae) surrounding the dorsal hollow nerve cord. The nerve cord is the one chordate feature present in the adult phase of all vertebrates. The vertebral column, part of a flexible but strong endoskeleton, is evidence that vertebrates are segmented. The vertebrate skeleton is living tissue (either cartilage or bone) that grows as the animal grows. The endoskeleton and muscles form an organ system (the musculoskeletal system) that permits rapid and efficient movement. The pectoral and pelvic fins of fishes evolved into jointed appendages that allowed vertebrates to move onto land. The skull, the most anterior component of the main axis of the vertebrate endoskeleton, encases the brain. The high degree of cephalization in vertebrates is accompanied by complex sense organs concentrated in the head region. Eyes developed as outgrowths of the brain. Ears were equilibrium devices in aquatic vertebrates that function as sound-wave receivers in land vertebrates. Vertebrates have a complete digestive system and a large coelom. Their circulatory system is closed, with respiratory pigments contained within blood vessels. Gas exchange is efficiently accomplished by gills, lungs, and in a few cases, moist skin. Kidneys are efficient in excretion of nitrogenous waste and regulation of water. Reproduction is usually sexual with separate sexes.
Classification of the Vertebrata
The first vertebrates were fishlike. Fishes are aquatic, gill-breathing vertebrates that usually have fins and skin covered with scales. The larval form of a modern-day lamprey, which looks like a lancelet, may resemble the first vertebrates: it has the three chordate characteristics (like the tunicate larva), as well as a two-chambered heart, a three-part brain, and other internal organs that are like those of vertebrates.
Small, jawless, and finless ostracoderms were the earliest vertebrates. They were filter feeders, but probably were also able to move water through their gills by muscular action. Ostracoderms have been found as fossils from the Cambrian through Devonian periods, when the group finally went extinct. Although extant jawless fishes lack protection, many early jawless fishes had large defensive head shields.
Class Petromyzontiformes, Lampreys
These long, eel-like, jawless fish are free-swimming predators on other fish. Lampreys hatch in freshwater and many live their lives entirely in freshwater. Some lampreys migrate to the sea, but must return to freshwater to reproduce. Lampreys have a sucker-like mouth that lacks a jaw. (Image is of sea lamprey on lake trout.)
Class Myxini, Hagfish
Members of the class Myxini have a partial cranium (skull), but no vertebrae. Their skeleton is made of cartilage, as is that of sharks. Hagfish lack jaws, and for this reason used to be classified with the lampreys in a group called the Agnatha ("no jaws") or the Cyclostomata ("round mouth").
Fish: Vertebrates With Jaws
The fish first appeared during the Cambrian Period. Whether fish first evolved in fresh or salt water is unclear from the fossil record. The jawless fish are the most primitive group, although they were a very important group during the Silurian and Devonian periods. Hagfish and lampreys are the only living members of this class today. They have long, cylindrical bodies with cartilage skeletons and no paired fins.
The first jawed fish were the Placoderms, an extinct group of Devonian-aged jawed fishes. Placoderms were armored with heavy plates and had strong jaws and paired pectoral and pelvic fins. Paired fins allow fish to balance and to maneuver well in water, which facilitate both predation and escape.
The evolution of jaws is an example of evolutionary modification of existing structures to perform new functions. Jaws are modified gill arches, and allowed the exploitation of new roles in the habitats: predators with powerful jaws. There are two classes of jawed fish: the cartilaginous fish and the bony fish.
Class Chondrichthyes: Cartilaginous Fish
The class Chondrichthyes contains approximately 850 species of skates, rays, and sharks. They have jaws, lots of teeth, paired fins, and a cartilage endoskeleton. Cartilaginous fish first appeared during the Devonian Period and expanded in diversity during the Carboniferous and Permian before nearly disappearing during the great extinction that occurred near the end of the Permian. A large group of cartilaginous fish still survives today and is an important part of the marine fauna. These fish have five to seven gill slits on both sides of the pharynx, and lack the gill covers found in bony fish. The chondrichthyian body is covered with toothlike scales. Developmental studies show the teeth of sharks are enlarged scales. The largest sharks are filter feeders, not the predators of Hollywood movies. Basking and whale sharks eat tons of crustaceans (small krills, etc.) filtered from the water. Most sharks are fast-swimming, open-sea predators. The great white shark feeds on dolphins, sea lions and seals (and people sometimes). In other words, anything is WANTS to!
Rays and skates live on the ocean floor; their pectoral fins are enlarged into winglike fins; they swim slowly. Stingrays have a venomous spine. The electric ray family can feed on fish that have been stunned with electric shock of over 300 volts. Sawfish rays have a large anterior "saw" that they use to slash through schools of fish.
Class Osteichthyes, the Bony Fish
There are about 20,000 species of bony fish, found both in marine and freshwater, comprising the class Osteichthyes. This class is divided into two groups: the lobe-finned (Sarcopterygii) and ray-finned fish (Actinopterygii). The bony fish have a bony skeleton. Most species in this class are ray-finned with thin, bony rays supporting the fins. A few fishes are lobe-finned and are thought to be related to the ancestors of amphibians.
The ray-finned fish include familiar species such as tuna, bass, perch, and trout. Ray-finned fish are the most successful and diverse of the vertebrates (more than half of all vertebrate species belong to this group). Thin, bony supports with radiating bones (hence the term ray-finned) hold the fins away from the body. Ray-finned fish obtain their food by filter feeding and by preying on insects and other animals. Their skin is covered by scales formed of bone. These scales are homologous to our own hair (and the feathers of birds), being derived from the same embryonic tissues. The gills in this group of fish do not open separately and are covered by an operculum. Ray-finned fish have a swim bladder, a gas-filled sac, that regulates buoyancy and depth. Sharks lack this feature, which enables fish to "sleep" without sinking. The swim bladder acts much the way a ballast tank does on a submarine to control buoyancy.
Salmon, trout, and eels can migrate from fresh water to salt water, but must adjust kidney and gill function to the tonicity of their environments. In freshwater, the fish is hypotonic relative to its aqueous (watery) environment. Water is constantly flooding into the fish, and must be removed by the fish's excretory system. In seawater, the fish is now hypertonic or isotonic relative to the seawater, requiring conservation of body water.
Bony fishes depend on color vision to detect both rivals and mates. Sperm and eggs are released into the water, with not much parental care for the newborn. Most fish have fertilization and embryonic development taking place outside the female's body.
This group includes six species of lungfishes and one species of coelacanth that has muscular fins with large, jointed bones attaching the fins to the body. Lobe-finned fish have fleshy fins supported by central bones, homologous to the bones in your arms and legs. These fins underwent modification, becoming the limbs of amphibians and their evolutionary descendants such as lizards, canaries, dinosaurs, and humans. The lungfish are a small group found mostly in freshwater stagnant water or ponds that dry up in Africa, South America, and Australia.
Coelacanths live in deep oceans. They were once considered extinct, although more than 200 have been captured since 1938. Mitochondrial DNA analysis supports the hypothesis that lungfish are probably the closest living relatives of amphibians.
The crossopterygian fish (represented by the marine extant deep-living coelacanth and extinct freshwater forms) are regarded as ancestors of early amphibians. Extinct crossopterygians had strong fins, lungs, and a streamlined body capable of swimming as well as traveling short distances out of water.
The term "tetrapod" (meaning four-limbed or four-footed) has historically been applied to the land vertebrates (amphibians, reptiles, dinosaurs, birds, and mammals). Most zoologists would accept that the Devonian lobe-finned fishes were ancestral to the amphibians. Animals (both vertebrate as well as many invertebrates such as insects) that live on land use limbs to support the body, especially since air is less buoyant than water. Lobe-finned fishes and early amphibians also had lungs and internal nares to respire air. Two hypotheses have been proposed to explain the evolution of amphibians from lobe-finned fishes.
1. Lobe-finned fishes capable of moving from pond-to-pond had an advantage over those that could not.
2. The supply of food on land, and the absence of predators, promoted adaptation to land.
Class Amphibia: Animals Move Ashore
This class includes 4000 species of animals that spend their larval/juvenile stages in water, and their adult life on land. Amphibians must return to water to mate and lay eggs. Most adults have moist skin that functions in helping their small, inefficient lungs with gas exchange. Frogs, toads, newts, salamanders, and mud puppies are in this transitional group between water and land. Amphibian features not seen in bony fish include:
* Limbs with girdles of bone that are adapted for walking on land.
* A tongue that can be used for catching prey as well as sensory input.
* A larynx adapted for vocalization.
* Eyelids that help keep the eyes moist.
* Ears adapted for detecting sound waves moving through the thin (as compared to water) medium of the air.
* A larger brain than that of fish, and a more developed cerebral cortex.
* Skin that is thin, smooth, nonscaly, and contains numerous mucous glands; the skin plays an active role in osmotic balance and respiration.
* Development of a lung that is permanently used for gas exchange in the adult form, although some amphibians supplement lung function by exchange of gases across a porous (moist) skin.
* A closed double-loop circulatory system that replaces the single-loop circulatory path of fish.
* Development of a three-chambered heart that pumps mixed blood before and after it has gone to the lungs.
Reproduction involves a return to the water. Ther term "amphibian" refers to two life styles, one in water, the other on land. Amphibians shed eggs into the water where external fertilization occurs, as it does in fish. Generally, amphibian eggs are protected by a coat of jelly but not by a shell. The young hatch into aquatic larvae with gills (tadpoles). Aquatic larvae usually undergo metamorphosis to develop into a terrestrial adult.
Amphibians, like fish, are ectothermic; they depend upon external heat to regulate body temperatures. If the environmental temperature becomes too low, ectotherms become inactive.
Salamanders more likely resemble earliest amphibians due to their S-shaped movements. Salamanders practice internal fertilization; males produce a spermatophore that females pick up. Frogs and toads are tailless as adults, with their hind limbs specialized for jumping.
Class Reptilia: Reproducton Without Water
This class of 6000 species includes the snakes, lizards, turtles, alligators, and crocodiles. Reptiles that lay eggs lay an egg surrounded by a thick protective shell and a series of internal membranes. Reptiles have internal fertilization: their gametes do not need to be released into water for fertilization to occur.
The amniotic egg is a superb adaptation to life on land. While amphibians need to lay their eggs in water, their descendants (reptiles) were not as strongly tied to moist environments and could truly expand into more arid areas. Reptiles were the first land vertebrates to practice internal fertilization through copulation and to lay eggs that are protected by a leathery shell with food and other support for the growing embryo.
The amniote egg contains extraembryonic membranes that are not part of the embryo and are disposed of after the embryo has developed and hatched. These membranes protect the embryo, remove nitrogenous wastes, and provide the embryo with oxygen, food, and water. The amnion, one of these extraembryonic membranes, creates a sac that fills with fluid and provides a watery environment in which the embryo develops. The embryo develops in a "pond within the shell".
Evolutionary History of Reptiles
Reptiles first evolved during the Carboniferous time and partly displaced amphibians in many environments. Reptilian success was due to their terrestrial (amniotic) egg and internal fertilization, as well as their tough leathery skin, more efficient teeth and jaws, and in some, bipedalism (traveling on their hind legs, allowing the forelimbs to grasp prey or food, or become wings). One group, the Pelycosaurs (fin-backed or sail lizards) are related to therapsids, mammal-like reptiles ancestral to mammals. Other groups returned to aquatic environments. Ichthyosaurs were fishlike (or dolphin-like) free-swimming predators of the Mesozoic seas. The plesiosaurs had a long neck and a body adapted to swimming though use of flippers (legs that evolutionarily reverted to a flipper-like shape). These free-swimmers also adapted to live birth of their young (since they could not return to the land to lay eggs). Thecodonts were the reptiles that gave rise to most of the reptiles, living and extinct. Pterosaurs were flying reptiles that dominated the Mesozoic skies. They had a keel for attachment of flight muscles and air spaces in bones to reduce weight. Dinosaurs (descended from some thecodonts) and mammal-like reptiles' had their limbs beneath the body providing increased agility and facilitating gigantic size. Lizards have their elbows out (like you do when you do a push-up).
Reptiles dominated the earth for about 170 million years during the Mesozoic Era. The mass extinction of many reptile groups at the close of the Mesozoic (the Cretaceous Period) has been well documented and the subject of many hypotheses. The 1980 hypothesis by Luis and Walter Alvarez and others proposes the impact of a large meteorite at the end of the Cretaceous period caused a catastrophic environmental collapse that led to the extinction of nearly 50% of all species of life on Earth. The survivors, birds and mammals, reaped the spoils and diversified during the Cenozoic Era. Three groups of reptiles remain: turtles, snakes/lizards, and crocodiles/alligators.
About 6,000 species of reptiles comprise the Class Reptilia. Most live in tropics or subtropics. Lizards and snakes live on land, while turtles and alligators live in water for much of their lives. Reptiles have a thick, scaly skin that is keratinized and impermeable to water. This same keratin is a protein found in hair, fingernails, and feathers. Protective skin prevents water loss but requires several molts a year. Reptilian lungs are more developed than those of amphibians. Air moves in and out of the lungs due to the presence of an expandable rib cage in all reptiles except turtles. Most reptiles have a nearly four-chambered heart. The crocodile has a completely four-chambered heart that more fully separates oxygen-rich blood from from deoxygenated or oxygen-poor blood. The well-developed kidneys excrete uric acid; less water is lost in excretion. Reptiles are ectothermic; they require a fraction of the food per body weight of birds and mammals, but are behaviorally adapted to warm their body temperature by sunbathing.
Snakes and lizards live mainly in the tropics and desert. Lizards have four clawed legs and are carnivorous; marine iguanas on the Galapagos are adapted to spend time in the sea; frilled lizards have a collar to scare predators, and blind worm lizards live underground. Snakes evolved from lizards and lost their legs as an adaptation to burrowing. Their jaws can readily dislocate to engulf large food. The snake's tongue collects airborne molecules and transfers them to the Jacobson's organ for tasting. Some poisonous snakes have special fangs for injecting their venom.
Turtles have a heavy shell fused to the ribs and thoracic vertebrae; they lack teeth but use a sharp beak; sea turtles must leave the ocean to lay eggs onshore.
Crocodiles and alligators are largely aquatic, feeding on fishes and other animals. They both have a muscular tail that acts as a paddle to swim and a weapon. The male crocodile bellows to attract mates. In some species the male also protects the eggs and young.
Class Aves: Birds of a Feather
The class Aves (birds) contains about 9000 species. Birds evolved from either a dinosaurian or other reptilian group during the Jurassic (or possibly earlier). The earliest bird fossils, such as the Jurassic Archaeopteryx or Triassic Protavis, display a mosaic of reptilian and bird features (teeth in the bill, a jointed tail, and claws on the wing are reptilian; feathers and hollow bones are bird-like). The distinguishing feature of birds is feathers: which provide insulation as well as aid in flight. Remember, not all animals that fly have feathers, but all almost every endothermic animal (warm-blooded) has a covering of hair or feathers for insulation. The recent (1999) discovery of a "feathered" dinosaur adds credence to this speculation. The dinosaur could not fly, so of what use would feather be but insulation (or possibly mating).
Modern birds have adapted to all modes of life: flying (condors, eagles, hummingbirds), flightless-running (ostriches, emus), and swimming (penguins). Birds exhibit complex mating rituals as well as social structure (a pecking order!).
Class Mammalia: Got Milk?
Class Mammalia contains around 5000 species placed in 26 orders (usually). The three unifying mammalian characteristics are:
2. the presence of three middle ear bones
3. the production of milk by mammary glands
Milk is a substance rich in fats and proteins. Mammary glands usually occur on the ventral surface of females in rows (when there are more than two glands). Humans and apes have two mammary glands (one right, one left), while other animals can have a dozen or more. All mammals have hair at some point during their life.
Mammalian hair is composed of the protein keratin. Hair has several functions: 1) insulation; 2) sensory function (whiskers of a cat);
3) camouflage, a warning system to predators, communication of social information, gender, or threats; and 4) protection as an additional layer or by forming dangerous spines that deter predators.
Modifications of the malleus and incus (bones from the jaw in reptiles) work with the stapes to allow mammals to hear sounds after they are transmitted from the outside world to their inner ears by a chain of these three bones.
Mammals first evolved from the mammal-like reptiles during the Triassic period, about the same time as the first dinosaurs. However, mammals were minor players in the world of the Mesozoic, and only diversified and became prominent after the extinction of dinosaurs at the close of the Cretaceous period.
Mammals have since occupied all roles once held by dinosaurs and their relatives (flying: bats; swimming: whales, dolphins; large predators: tigers, lions; large herbivores: elephants, rhinos), as well as a new one (thinkers and tool makers: humans). There are 4500 species of living mammals.
Mammals developed several adaptations that help explain their success.
* Teeth are specialized for cutting, shearing or grinding; thick enamel helps prevent teeth from wearing out.
* Mammals are capable of rapid locomotion.
* Brain sizes are larger per pound of body weight than most other animals'.
* Mammals have more efficient control over their body temperatures than do birds.
* Hair provides insulation.
* Mammary glands provide milk to nourish the young.
Subclass Prototheria: Order Monotremata: Monotremes (typified by the platypus and echinda) lay eggs that have similar membranes and structure to reptilian eggs. Females burrow in ground and incubate their eggs. Both males and females produce milk to nourish the young There are two families living today and quite a few known from the fossil record of Gondwana. Monotremes are today restricted to Australia and New Guinea. The earliest fossil monotreme is from the early Cretaceous, and younger fossils hint at a formerly more widespread distribution for the group. While their fossil record is scarce, zoologists believe that monotremes probably diverged from other mammals during the Mesozoic. Monotremes have many differences with other mammals and are often placed in a separate group, the subclass Prototheria. They retain many characters of their therapsid ancestors, such as laying eggs, limbs oriented with humerus and femur held lateral to body (more lizard-like), a cloaca, skulls with an almost birdlike appearance, and a lack of teeth in adults.
Subclass Metatheria: Marsupials (such as the koala, opossum, and kangaroo) are born while in an embryonic stage and finish development outside the mother's body, often in a pouch. Marsupial young leave the uterus, crawl to the pouch, and attach to the nipple of a mammary gland and continue their development. Marsupials were once widespread, but today are dominant only in Australia, where they underwent adaptive radiation in the absence of placental mammals. The Metatheria contains 272 species classified in several orders. Australian marsupials remain diverse and dominant native mammals of the fauna.
Subclass Eutheria: There are 4000 described species of placental mammals, a group that includes dogs, cats, and people. The subclass is defined by a true placenta that nourishes and protects the embryos held within the mother's body for an extended gestation period (nearly two years for an elephant, and nine months for a human). The eutherian placenta has extraembryonic membranes modified for internal development within the uterus. The chorion is the fetal portion of placenta, while the uterine wall grows the maternal portion. The placenta exchanges nutrients, oxygen, and wastes between fetal and maternal blood.
There are 12 orders of placental mammals. Classification is based on the mode of locomotion and methods of obtaining food. Prominent orders include the bats (order Chiroptera), horses (order Perissodactyla), whales (order Cetacea), mice (order Rodentia), dogs (order Carnivora), and monkeys/apes/humans (order Primates).