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1. Explain the classification and the phylogeny of each phyla.

Despite their very different appearances such familiar invertebrates as chitons, clams, snails, and octopuses are all mollusks, members of the phylum mollusca. The word Mollusk comes from a Latin term meaning "soft" and refers to the bodies of these animals. Although slugs and octopuses have soft bodies, most mollusks have a shell that protects and conceals their soft body.

Relationship of Mollusca and Annelida

Based on cooperative studies of living species, biologists have concluded that members of the Phylum Mollusca probably share a common ancestor with the segmented worms of the Phylum Annelida. Mollusks and annnelids have similar patterns of embryonic development. In addition, mollusks and annelids were probably the first major groups of organisms to have a true coelom - that is, a fluid filled cavity within the mesoderm. The coelom provides several benefits. By separating the muscles of the gut from those of the body wall, the coelom allows food to move through the body independent of locomotion. The coelom also provides a space in which a circulatory system can function without interference from other organs. In some species the fluid in the coelom also forms a sort of internal hydrostatic skeleton against which the muscles can contract.

The strongest evidence for the common ancestry of mollusks and annelids is the characteristic larval form they share. In both groups the first stage of larval development is a pear-shaped larva called trochophore. Cilia project from both ends of the trochophore and circle the middle. These cilia propel the trochophore through the water and draw food top the mouth. Free-swimming trochophores aid in the dispersal of many marine mollusks. However, in terrestrial mollusks and in many marine annelids, the trochophore develops within the egg and is not free-living.

Characteristics of mollusks

Mollusks are a diverse group of more than 100,000 species. Among animal phyla, only Arthropoda has more species. Some mollusks are sedentary filter feeders, such as clams and oysters. Others are predatory, such as squids and octopuses - which move about by jet propulsion and have complex nervous systems. Mollusks also have diverse connections with humans. Certain species of snails, for example, are alternate hosts of parasites, such as schistosomes, that are harbored by humans. The feeding habits of snails and slugs cause damage to crops. In contrast, humans prize many mollusks as food or for the beauty of their shells.

Mollusks all have the following characteristics:

• All Mollusks have a true coelom.

• The body has three distinct parts: the muscular foot, the head, and the visceral mass.

• Mollusks have organ systems for circulation, respiration, digestion, excretion, nerve impulse conduction, and reproduction.

• Most Mollusks are bilaterally symmetrical and have one or more shells.

Body plan

Mollusk species have numerous variations of the same basic body plan. All Mollusks have: the muscular foot, the head and the visceral mass. The muscular foot is a large organ used in locomotion. The head contains the mouth, sense organs and cerebral ganglia. The visceral mass contains the hearth and the organs of digestion, excretion and reproduction. Covering the visceral mass is the mantle, an epidermal layer that in most species secretes a shell.

Organisms with a hard shell of calcium carbonate probably appeared early in the evolution of mollusks. The shell was of an adaptive value to these organisms because it protected the soft body from predators. However, it also conferred a disadvantage since shell reduced the surface area available for gas exchange. This problem was avoided as a later structural adaptation evolved - the gills. With a large surface area and a rich supply of blood, gills are organs specialized for the exchange of gases with water. The delicate gills are protected within the mantle cavity, a space between the mantle and the visceral mass.

Biologists use structural differences to classify mollusks into four to seven classes. The four major classes are: Polyplacophora, Gastropoda, Bivalvia, and Cephalopoda.

Kingdom Animalia

Phylum Mollusca: Coelomates; undergo spiral cleavage in early stages of development; body in three parts; all organ systems present; mantle, secreting a shell in many genera; rasplike radula; trochophore and veliger larvae; terrestrial, freshwater, marine: clams, snails, octopuses.

Class Gastropoda: Stomach-footed, with or without single coiled shell; head, distinct eyes, and tentacles present: snail, slug, whelk

Class Bivalvia: Hatched-footed with bivalve shell; gills in mantle cavity; head, eyes, and tentacles lacking: clam, oyster, scallop.

Class Cephalopoda: Head-footed; foot modified into grasping tentacles; marine animals; squid, octopus, chambered nautilus

Class Polyplacophora: Elongate body and reduced head, without tentacles; many forms with a shell composed of eight plates: chitin

Class Scaphopoda: Body elongated and enclosed in a tubular shell, open at both ends; gills lacking; marine: tooth shells

2. Describe the characteristics typical for the diversity shown by each phyla.

Despite their very different appearances such familiar invertebrates as chitons, clams, snails, and octopuses are all mollusks, members of the phylum mollusca. The word Mollusk comes from a Latin term meaning "soft" and refers to the bodies of these animals. Although slugs and octopuses have soft bodies, most mollusks have a shell that protects and conceals their soft body.

Relationship of Mollusca and Annelida

Based on cooperative studies of living species, biologists have concluded that members of the Phylum Mollusca probably share a common ancestor with the segmented worms of the Phylum Annelida. Mollusks and annnelids have similar patterns of embryonic development. In addition, mollusks and annelids were probably the first major groups of organisms to have a true coelom - that is, a fluid filled cavity within the mesoderm. The coelom provides several benefits. By separating the muscles of the gut from those of the body wall, the coelom allows food to move through the body independent of locomotion. The coelom also provides a space in which a circulatory system can function without interference from other organs. In some species the fluid in the coelom also forms a sort of internal hydrostatic skeleton against which the muscles can contract.

The strongest evidence for the common ancestry of mollusks and annelids is the characteristic larval form they share. In both groups the first stage of larval development is a pear-shaped larva called trochophore. Cilia project from both ends of the trochophore and circle the middle. These cilia propel the trochophore through the water and draw food top the mouth. Free-swimming trochophores aid in the dispersal of many marine mollusks. However, in terrestrial mollusks and in many marine annelids, the trochophore develops within the egg and is not free-living.

Characteristics of mollusks

Mollusks are a diverse group of more than 100,000 species. Among animal phyla, only Arthropoda has more species. Some mollusks are sedentary filter feeders, such as clams and oysters. Others are predatory, such as squids and octopuses - which move about by jet propulsion and have complex nervous systems. Mollusks also have diverse connections with humans. Certain species of snails, for example, are alternate hosts of parasites, such as schistosomes, that are harbored by humans. The feeding habits of snails and slugs cause damage to crops. In contrast, humans prize many mollusks as food or for the beauty of their shells.

Mollusks all have the following characteristics:

• All Mollusks have a true coelom.

• The body has three distinct parts: the muscular foot, the head, and the visceral mass.

• Mollusks have organ systems for circulation, respiration, digestion, excretion, nerve impulse conduction, and reproduction.

• Most Mollusks are bilaterally symmetrical and have one or more shells.

Body plan

Mollusk species have numerous variations of the same basic body plan. All Mollusks have: the muscular foot, the head and the visceral mass. The muscular foot is a large organ used in locomotion. The head contains the mouth, sense organs and cerebral ganglia. The visceral mass contains the hearth and the organs of digestion, excretion and reproduction. Covering the visceral mass is the mantle, an epidermal layer that in most species secretes a shell.

Organisms with a hard shell of calcium carbonate probably appeared early in the evolution of mollusks. The shell was of an adaptive value to these organisms because it protected the soft body from predators. However, it also conferred a disadvantage since shell reduced the surface area available for gas exchange. This problem was avoided as a later structural adaptation evolved - the gills. With a large surface area and a rich supply of blood, gills are organs specialized for the exchange of gases with water. The delicate gills are protected within the mantle cavity, a space between the mantle and the visceral mass.

Biologists use structural differences to classify mollusks into four to seven classes. The four major classes are: Polyplacophora, Gastropoda, Bivalvia, and Cephalopoda.

Polyplacophora

Members of the Class Polyplacophora are called chitons. These are primarily animals of the seashore that live on rocks. The word Polyplacophora means "many plates", a reference to the main characteristic of the class: their shells are divided into eight separate, overlapping plates.

Gastropoda

The largest and most diverse class of mollusks is Gastropoda, which includes about 75,000 species of snails, slugs, abalones, nudibranchs, and conches. The name gastropoda means "stomach-foot". Most gastropods have a single shell, or valve, and so are called univalves. Slugs and some other gastropods have no shell.

The body plan of gastropods is based on that of ancestral mollusks. The major difference results from torsion, or twisting, which occurs during larval development. In torsion the visceral mass twists 180 degrees in relation to the head. The twisting brings the mantle cavity to the front of the animal, thus allowing the animal to draw its head into the mantle cavity when endangered.

Snails

Snails are gastropods that live in a wide range of environments as well as in fresh water and oceans. Aquatic snails respire through gills in the mantle cavity. In terrestrial snails the mantle cavity acts as a modified lung that allows the animal to obtain oxygen from the air. The thin membrane that lines the cavity must be kept moist to allow gases to diffuse through it. For this reason snails are most active when the air has a higher moisture content. During dry periods a snail survives by becoming inactive. It retreats into its shell and seals the opening with a mucus plug, which keeps the snail from drying out.

Like all gastropods, snails have an open circulatory system, meaning that the blood does not circulate entirely within vessels. Instead blood is collected from the gills or lungs, pumped through the hearth, and released directly into spaces in the tissues. From there it returns via the gills or lungs to the heart. The blood-filled space is known as a hemocoel or blood cavity.

The main feeding adaptation of snails is the sawlike radula, a flexible tongue strip covered with chitinous teeth. Aquatic snails use the radula to scrape up algae or other food. Terrestrial snails use the radula to saw off leaves or garden plants.

Land snails are hermaphroditic, but in most aquatic species the sexes are distinct. Eggs are fertilized internally.

Snails move smoothly by the wavelike muscular contractions of the foot. Glands in the foot secrete a layer of mucus on which the animal travels at a pace of about 3m per hour. Two eyes at the end of delicate tentacles on the head help locate food. If danger should arise, the tentacles retract into the head.

Other Gastropods

Slugs look like snails without shells. They survive without shells because they live in moist environments, hiding in shady places by day and feeding at night. Like terrestrial snails, slugs respire through the lining of the mantle cavity.

Some gastropods show unusual modifications of the foot or radula. Instead of having a foot, pteropods, or "sea butterflies", have a winglike flap, which allows them to swim on the surface of the sea. Another adaptation is seen in gastropods with modified radulas. Oyster drills, for example, use their radulas to bore through oysters shells and feed on the soft tissue inside.

Bivalvia

In contrast with gastropods that move about in search of food, most members of the Class Bivalvia are sessile and filter food from the water. Bivalvia means "two valves". Clams, oysters, scallops, and other bivalves have a shell with two valves and a muscular foot. Once this foot is in sand, blood swells the end, causing it to spread and form a hatchet-shaped anchor. The muscles of the foot then contract and pull the animal down in the sand. Bivalves do not have distinct head regions as do gastropods. As an adaptation for sessile existence, ganglia are present in the anterior region.

Each valve consists of three layers secreted by the mantle. A thin outer layer protects the shell against acidic conditions in the water. A thick middle layer of calcium carbonate crystals strengthens the shell. The smooth, iridescent inner layer protects the animal’s soft body. If an irritant such as grain of sand gets inside the shell, the mantle coats it with a secretion known as mother of pearl. Layers of this secretion form a pearl. A hinge connects the two valves of the shell. The animal can close the valves with its powerful adductor muscles. When the adductor muscles relax, the valves open.

Clams

Clams are bivalves that live buried in mud, seashore sand, or sand on the bottom of the sea. Clams have evolved adaptations for filter feeding. The mantle cavity is sealed except for a pair of hollow tubes called siphons. Cilia beating on the gills set up a current that causes water to enter the incurrent siphon. Cilia then propel the water over the gills and cause it to exit through the excurrent siphon. As the cilia move water over the gills, food - plankton from the water and organic sediments from the sea bottom - becomes trapped in a sticky mucus. Then the cilia move this mucus to the mouth. As water passes over the gills, oxygen diffuses into the blood, and carbon dioxide diffuses out.

Clams have rudimentary sense organs. Sensory cells along the edge of the mantle respond to light and touch. Like most mollusks the ganglia that are above the mouth, in the digestive system, and in the foot are connected by two pairs of long nerve cords.

Most species of clams have separate sexes. They reproduce by shedding sperm and eggs into the water, and fertilization occurs externally. The fertilized egg becomes a trochophore larva that eventually settles to the bottom and develops into an adult.

Other Bivalves

Oysters are bivalves that become permanently attached to a hard surface early in their development. Scallops are bivalves that move through the water by opening their valves and then rapidly snapping them shut. This motion expels bursts of water, creating a form of jet propulsion. The teredo, or shipworm, is one of the few bivalves that does not filter feed. Instead, it bores into driftwood or ship timbers. Particles produced by the drilling are ingested by the teredo. The cellulose is broken down by symbiotic protozoa that live in the shipworm’s intestine.

Cephalopoda

Cephalopods such as octopuses, squids, cuttlefish, and chambered nautiluses are evolutionarily advanced and complex mollusks. The name cephalopod, meaning "head-foot", refers to the large, well-developed head and the prominent foot divided into numerous tentacles. The head and foot are specialized for a free-swimming, predatory existence. Cephalopods capture pray with powerful tentacles equipped with strong suckers. They kill and eat their prey with the help of a radula and a sharp beak.

Cephalopods have a closed circulatory system, in which blood circulates entirely within a system of vessels. This efficient circulation rapidly transports the food and oxygen in these highly active animals

Squids

Like all cephalopods, squids live in oceans. Most grow to about 30 cm in length, but a few species grow much longer. The giant squid may grow to 20 m long and weight more then 3360 kg. It is the world’s largest known invertebrate.

A squid’s head encloses a large, complex brain that controls a highly developed nervous system. The squid’s most prominent sense organs are it pair of large eyes, similar in structure to those of humans. Squids have ten tentacles. The longest pair is used for capturing pray; the smaller pairs force the pray into the mouth. The mascular mantle propels the squid by pumping jets of water through an excurrent siphon.

Squids have defensive adaptations. They can squirt an inky substance at enemies. The substance temporarily blinds the enemy and dulls its sense of smell. Pigment cells called chromatophores, located on the outer layer of the mantle, enable squids to change color and blend in with their surroundings.

Like all cephalopods, squids have separate sexes. The male uses a specialized tentacle to transfer packets of sperm from its mantle cavity to the cavity of the female, where fertilization occurs. The female lays a mass of fertilized eggs encased in a gelatinous material. She then guards them until they hatch.

Other Cephalopods

Octopuses seldom grow to more then 30 cm in diameter. They have eight tentacles and resemble squids in the ways they move, feed, capture pray, and escape from enemies. Instead of using jet propulsion to chase prey, they may crawl along the bottom with their tentacles or lie in wait in caves and rock crevices. The jet propulsion may be used to escape danger.

The chambered nautilus is the only existing cephalopod that has retained its exterior shell. The nautilus lives in an outer chamber of its shell and secretes a gas into the other chambers, the nautilus can adjust its buoyancy and so control its depth in water.

3. Describe the life cycles typical of organisms in this phylum.

Polyplacophora

Members of the Class Polyplacophora are called chitons. These are primarily animals of the seashore that live on rocks. The word Polyplacophora means "many plates", a reference to the main characteristic of the class: their shells are divided into eight separate, overlapping plates.

Gastropoda

The largest and most diverse class of mollusks is Gastropoda, which includes about 75,000 species of snails, slugs, abalones, nudibranchs, and conches. The name gastropoda means "stomach-foot". Most gastropods have a single shell, or valve, and so are called univalves. Slugs and some other gastropods have no shell.

The body plan of gastropods is based on that of ancestral mollusks. The major difference results from torsion, or twisting, which occurs during larval development. In torsion the visceral mass twists 180 degrees in relation to the head. The twisting brings the mantle cavity to the front of the animal, thus allowing the animal to draw its head into the mantle cavity when endangered.

Snails

Snails are gastropods that live in a wide range of environments as well as in fresh water and oceans. Aquatic snails respire through gills in the mantle cavity. In terrestrial snails the mantle cavity acts as a modified lung that allows the animal to obtain oxygen from the air. The thin membrane that lines the cavity must be kept moist to allow gases to diffuse through it. For this reason snails are most active when the air has a higher moisture content. During dry periods a snail survives by becoming inactive. It retreats into its shell and seals the opening with a mucus plug, which keeps the snail from drying out.

Like all gastropods, snails have an open circulatory system, meaning that the blood does not circulate entirely within vessels. Instead blood is collected from the gills or lungs, pumped through the hearth, and released directly into spaces in the tissues. From there it returns via the gills or lungs to the heart. The blood-filled space is known as a hemocoel or blood cavity.

The main feeding adaptation of snails is the sawlike radula, a flexible tongue strip covered with chitinous teeth. Aquatic snails use the radula to scrape up algae or other food. Terrestrial snails use the radula to saw off leaves or garden plants.

Land snails are hermaphroditic but in most aquatic species the sexes are distinct. Eggs are fertilized internally.

Snails move smoothly by the wavelike muscular contractions of the foot. Glands in the foot secrete a layer of mucus on which the animal travels at a pace of about 3m per hour. Two eyes at the end of delicate tentacles on the head help locate food. If danger should arise, the tentacles retract into the head.

Other Gastropods

Slugs look like snails without shells. They survive without shells because they live in moist environments, hiding in shady places by day and feeding at night. Like terrestrial snails, slugs respire through the lining of the mantle cavity.

Some gastropods show unusual modifications of the foot or radula. Instead of having a foot, pteropods, or "sea butterflies", have a winglike flap, which allows them to swim on the surface of the sea. Another adaptation is seen in gastropods with modified radulas. Oyster drills, for example, use their radulas to bore through oysters shells and feed on the soft tissue inside.

Bivalvia

In contrast with gastropods that move about in search of food, most members of the Class Bivalvia are sessile and filter food from the water. Bivalvia means "two valves". Clams, oysters, scallops, and other bivalves have a shell with two valves and a muscular foot. Once this foot is in sand, blood swells the end, causing it to spread and form a hatchet-shaped anchor. The muscles of the foot then contract and pull the animal down in the sand. Bivalves do not have distinct head regions as do gastropods. As an adaptation for sessile existence, ganglia are present in the anterior region.

Each valve consists of three layers secreted by the mantle. A thin outer layer protects the shell against acidic conditions in the water. A thick middle layer of calcium carbonate crystals strengthens the shell. The smooth, iridescent inner layer protects the animal’s soft body. If an irritant such as grain of sand gets inside the shell, the mantle coats it with a secretion known as mother of pearl. Layers of this secretion form a pearl. A hinge connects the two valves of the shell. The animal can close the valves with its powerful adductor muscles. When the adductor muscles relax, the valves open.

Clams

Clams are bivalves that live buried in mud, seashore sand, or sand on the bottom of the sea. Clams have evolved adaptations for filter feeding. The mantle cavity is sealed except for a pair of hollow tubes called siphons. Cilia beating on the gills set up a current that causes water to enter the incurrent siphon. Cilia then propel the water over the gills and cause it to exit through the excurrent siphon. As the cilia move water over the gills, food - plankton from the water and organic sediments from the sea bottom - becomes trapped in a sticky mucus. Then the cilia move this mucus to the mouth. As water passes over the gills, oxygen diffuses into the blood, and carbon dioxide diffuses out.

Clams have rudimentary sense organs. Sensory cells along the edge of the mantle respond to light and touch. Like most mollusks the ganglia that are above the mouth, in the digestive system, and in the foot are connected by two pairs of long nerve cords.

Most species of clams have separate sexes. They reproduce by shedding sperm and eggs into the water, and fertilization occurs externally. The fertilized egg becomes a trochophore larva that eventually settles to the bottom and develops into an adult.

Other Bivalves

Oysters are bivalves that become permanently attached to a hard surface early in their development. Scallops are bivalves that move through the water by opening their valves and then rapidly snapping them shut. This motion expels bursts of water, creating a form of jet propulsion. The teredo, or shipworm, is one of the few bivalves that does not filter feed. Instead, it bores into driftwood or ship timbers. Particles produced by the drilling are ingested by the teredo. The cellulose is broken down by symbiotic protozoa that live in the shipworm’s intestine.

Cephalopoda

Cephalopods such as octopuses, squids, cuttlefish, and chambered nautiluses are evolutionarily advanced and complex mollusks. The name cephalopod, meaning "head-foot", refers to the large, well-developed head and the prominent foot divided into numerous tentacles. The head and foot are specialized for a free-swimming, predatory existence. Cephalopods capture pray with powerful tentacles equipped with strong suckers. They kill and eat their prey with the help of a radula and a sharp beak.

Cephalopods have a closed circulatory system, in which blood circulates entirely within a system of vessels. This efficient circulation rapidly transports the food and oxygen in these highly active animals

Squids

Like all cephalopods, squids live in oceans. Most grow to about 30 cm in length, but a few species grow much longer. The giant squid may grow to 20 m long and weight more then 3360 kg. It is the world’s largest known invertebrate.

A squid’s head encloses a large, complex brain that controls a highly developed nervous system. The squid’s most prominent sense organs are it pair of large eyes, similar in structure to those of humans. Squids have ten tentacles. The longest pair is used for capturing pray; the smaller pairs force the pray into the mouth. The mascular mantle propels the squid by pumping jets of water through an excurrent siphon.

Squids have defensive adaptations. They can squirt an inky substance at enemies. The substance temporarily blinds the enemy and dulls its sense of smell. Pigment cells called chromatophores, located on the outer layer of the mantle, enable squids to change color and blend in with their surroundings.

Like all cephalopods, squids have separate sexes. The male uses a specialized tentacle to transfer packets of sperm from its mantle cavity to the cavity of the female, where fertilization occurs. The female lays a mass of fertilized eggs encased in a gelatinous material. She then guards them until they hatch.

Other Cephalopods

Octopuses seldom grow to more then 30 cm in diameter. They have eight tentacles and resemble squids in the ways they move, feed, capture pray, and escape from enemies. Instead of using jet propulsion to chase prey, they may crawl along the bottom with their tentacles or lie in wait in caves and rock crevices. The jet propulsion may be used to escape danger.

The chambered nautilus is the only existing cephalopod that has retained its exterior shell. The nautilus lives in an outer chamber of its shell and secretes a gas into the other chambers, the nautilus can adjust its buoyancy and so control its depth in water.

4. List all the animals which are used as examples in class.

Phylum Mollusca

Class Caudofoveata. Chaetoderma

Class Solenogastres. Neomenia, Proneomenia

Class Polyplacophora. Chitons- Chaetopleura, Ischnochiton, Lepidochiton, Amicula

Class Monoplacophora. Neopilina

Class Gastropoda. Snails and their allies( univalve mollusks ) - Helix, Busycon, Crepidula, Haliotis, Littorina, Doris, Limax.

Class Scaphoda. Tusk shells - Dentalium, Cadulus

Class Bivalva. Bivalve mollusks - Mytilus, Ostrea, Pecten, Mercenaria, Teredo, Tagelus, Unio, Anodonta

Class Cephalopoda. Squids, octopuses, etc... - Loligo, Octopus, Nautilus

5. Make a vocabulary list and briefly define them. Use all words printed in bold print.

1. Trochophore - the first stage of larval development in the groups of Mollusca and Annelida, a pear-shaped larva

2. Foot - a large organ in mollusks used for locomotion

3. Head - the part of mollusk which contains the mouth, sense organs, and cerebral ganglia

4. Visceral mass - the part of mollusk which contains the hearth and the organs of digestion, excretion, and reproduction.

5. Mantle - an epidermal layer that in most species secretes a shell, covering the visceral mass

6. Gills - organs specialized for the exchange of gases with water

7. Mantle cavity - a space between the mantle and the visceral mass, that protects the delicate gills

8. Torsion - the twisting of the visceral mass 180 degrees in relation to the head, that occurs during the larval development

9. Open circulatory system - a circulatory system where blood does not circulate entirely within vessels.

10. Hemocoel - the blood filled space of snail, or blood cavity

11. Radula - a flexible tonguelike strip covered with chitenous teeth

12. Incurrent siphon - The opening through which water enters the shell of a clam

13. Excurrent siphon - The opening through which water leaves the shell of a clam

14. Closed circulatory system - a circulatory system in which the blood circulates entirely within a system of vessels

15. Chromatophores - Pigment cells located on the outer layer of the mantle, that enable squids to change color and blend in with their surroundings.

6. List the special adaptations which make this phylum successful on earth.

Despite their very different appearances such familiar invertebrates as chitons, clams, snails, and octopuses are all mollusks, members of the phylum mollusca. The word Mollusk comes from a Latin term meaning "soft" and refers to the bodies of these animals. Although slugs and octopuses have soft bodies, most mollusks have a shell that protects and conceals their soft body.

Relationship of Mollusca and Annelida

Based on cooperative studies of living species, biologists have concluded that members of the Phylum Mollusca probably share a common ancestor with the segmented worms of the Phylum Annelida. Mollusks and annnelids have similar patterns of embryonic development. In addition, mollusks and annelids were probably the first major groups of organisms to have a true coelom - that is, a fluid filled cavity within the mesoderm. The coelom provides several benefits. By separating the muscles of the gut from those of the body wall, the coelom allows food to move through the body independent of locomotion. The coelom also provides a space in which a circulatory system can function without interference from other organs. In some species the fluid in the coelom also forms a sort of internal hydrostatic skeleton against which the muscles can contract.

The strongest evidence for the common ancestry of mollusks and annelids is the characteristic larval form they share. In both groups the first stage of larval development is a pear-shaped larva called trochophore. Cilia project from both ends of the trochophore and circle the middle. These cilia propel the trochophore through the water and draw food top the mouth. Free-swimming trochophores aid in the dispersal of many marine mollusks. However, in-terrestrial mollusks and in many marine annelids, the trochophore develops within the egg and is not free-living.

Characteristics of mollusks

Mollusks are a diverse group of more than 100,000 species. Among animal phyla, only Arthropoda has more species. Some mollusks are sedentary filter feeders, such as clams and oysters. Others are predatory, such as squids and octopuses - which move about by jet propulsion and have complex nervous systems. Mollusks also have diverse connections with humans. Certain species of snails, for example, are alternate hosts of parasites, such as schistosomes, that are harbored by humans. The feeding habits of snails and slugs cause damage to crops. In contrast, humans prize many mollusks as food or for the beauty of their shells.

Mollusks all have the following characteristics:

• All Mollusks have a true coelom.

• The body has three distinct parts: the muscular foot, the head, and the visceral mass.

• Mollusks have organ systems for circulation, respiration, digestion, excretion, nerve impulse conduction, and reproduction.

• Most Mollusks are bilaterally symmetrical and have one or more shells.

Body plan

Mollusk species have numerous variations of the same basic body plan. All Mollusks have: the muscular foot, the head and the visceral mass. The muscular foot is a large organ used in locomotion. The head contains the mouth, sense organs and cerebral ganglia. The visceral mass contains the hearth and the organs of digestion, excretion and reproduction. Covering the visceral mass is the mantle, an epidermal layer that in most species secretes a shell.

Organisms with a hard shell of calcium carbonate probably appeared early in the evolution of mollusks. The shell was of an adaptive value to these organisms because it protected the soft body from predators. However, it also conferred a disadvantage since shell reduced the surface area available for gas exchange. This problem was avoided as a later structural adaptation evolved - the gills. With a large surface area and a rich supply of blood, gills are organs specialized for the exchange of gases with water. The delicate gills are protected within the mantle cavity, a space between the mantle and the visceral mass.

Biologists use structural differences to classify mollusks into four to seven classes. The four major classes are: Polyplacophora, Gastropoda, Bivalvia, and Cephalopoda.

Kingdom Animalia

Phylum Mollusca: Coelomates; undergo spiral cleavage in early stages of development; body in three parts; all organ systems present; mantle, secreting a shell in many genera; rasplike radula; trochophore and veliger larvae; terrestrial, freshwater, marine: clams, snails, octopuses

Class Gastropoda: Stomach-footed, with or without single coiled shell; head, distinct eyes, and tentacles present: snail, slug, whelk

Class Bivalvia: Hatched-footed with bivalve shell; gills in mantle cavity; head, eyes, and tentacles lacking: clam, oyster, scallop.

Class Cephalopoda: Head-footed; foot modified into grasping tentacles; marine animals; squid, octopus, chambered nautilus

Class Polyplacophora: Elongate body and reduced head, without tentacles; many forms with a shell composed of eight plates: chitin

Class Scaphopoda: Body elongated and enclosed in a tubular shell, open at both ends; gills lacking; marine: tooth shells