Phylum Mollusca

By: Charmaigne Molina

Phylum Mollusca is the second largest of all animal phyla; it includes between 50, 000 and 110,000 living species, depending upon who is doing the counting. It came from the Greek word mollusca which mean “soft”.

Defining Characteristics:

1. Dorsal Epithelium forming a mantle, which secretes calcareous spicules or one or more shells;

2. Cuticular band of teeth called (radula) in the esophagus, used for feeding (not present—lost?—in bivalves);

3. Ventral body wall muscles develop in to a locomotory or clinging foot

These species are distributed among some extremely dissimilar looking organism, making the molluscan body plan probably the most malleable in the animal kingdom. There is really no “typical” mollusc. Most but not all, molluscs have shells consisting primarily of calcium carbonate set in a protein matrix. Organic material may comprise about 35% of the shell’s dry weight in

some gastropod species and up to about 70% of the dry weight in bivalves. This very diverse group includes chitons, tusk shells, snails, slugs, nudibranches, sea

butterflies, clams, mussels, oysters, squids, octopuses, and nautiluses. The group ranges from fairly simple organisms to some of the most complex of invertebrates; sizes range from almost microscopic to the giant squid Architeuthis .

Feeding• Herbivorous, only eat plants• Predaceous Carnivores • Filter Feeder• Detritus Feeders (feed on decayed materials)• Parasites (feed on other live organisms)

HabitatMolluscs are found in a great range of habitats, from the tropics to polar seas, at altitudes

exceeding 7000 m, in ponds, lakes, and streams, on mud flats, in pounding surf, and in open ocean from the surface to abyssal depths. They represent a various lifestyles, including bottom feeders, burrowers, borers, pelagic forms.

According to fossil evidence, mollusc originated in the sea, and most of them have remained there. Much of their evolution occurred along the shores, where food was abundant and habitats were varied. Only bivalves and gastropods moved into brackish and freshwater habitats. As filter

feeders, bivalves were unable to leave aquatic surroundings. Only slugs and snails (gastropods) actually invaded the land. Terrestrial snails are limited in their range by their need for humidity, shelter, and presence of calcium in the soil.

Humans and Molluscs

Humans exploit molluscs in a variety of ways. Many kinds of molluscs are used as food (mussels, clams, oysters, abalone, calamari (squid), octopus, escargot (snails), etc). Pearls are formed in the shell of bivalves. Shiny inner layer of some shells used to make buttons.

“If a grain of sand, parasite or other foreign particle becomes trapped between the mantle and the shells’ inner surface, a pearl may form over a period of years. Natural pearl formation is a fairly rare event; perhaps only one oyster in 1,000 is likely to harbor a valuable pearl naturally”

Human increase the frequency of pearl production by surgically implanting pieces of shell (usually from freshwater bivalves) or plastic spheres between the shell and mantle of mature oysters, and then keeping the oysters alive and protected for 5 to 7 years. Note that cultured pearl form in a perfectly normal manner; humans only intervene only in getting the process started.

Some molluscs are considered pests because of the damage they cause. Burrowing shipworms, which are bivalves of several species, do great damage to wooden ships and wharves. To prevent the ravages of shipworms, wharves must be creosote or built of concrete (unfortunately, some shipworms ignore creosote, and some bivalves bore into concrete).

Snails and slugs frequently damage the garden and other vegetation. In addition, snails often serve as intermediate hosts for serious parasites of humans and domestic animals. Boring snails of genus Urosalpinx rival sea star in destroying oysters.

FORM AND FUNCTION

Reduced to its simplest dimension, the mollusc body plan may be said to consist of head-foot portion and a visceral mass portion. The head-foot is the more active area, containing the feeding, cephalic sensory and locomotor organs. The visceral mass is the portion containing digestive, circulatory, respiratory, and reproductive organs.

Head-foot

It depends primarily on muscular action for its function. Most molluscs have well-developed heads, which bear their mouth and some specialized sensory organs

Radula

The radula consists of a firm ribbon, composed of chitin and protein, along which are found 2 rows of sharp, chitinous teeth. It is rasping, protrusible, tonguelike organ found in all molluscs except bivalves and most solenogaster. It is a ribbon-like membrane on which are mounted rows of tiny teeth

that point backward. The ribbon is produced from a radular sac. Complex muscles move the radula and its supporting cartilages (odontophore; G: tooth bearer) in and out while the membrane is partly rotated over the tips of the cartilages. There may be few or as many as 250,000 teeth, which, when protruded, can scrape, pierce, tear, or cut.

The odontophore-radular assembly, together with its complex musculature, is known as buccal mass (bucca= L: cheek), or the odontophore complex. The usual function of the radula is twofold: to rasp of the fine particles of food particles and to serve as conveyor belt for carrying particles in a continuous stream toward the digestive tract.

As old teeth are worn down or broken off at the anterior end of the radular ribbon, new teeth are continually being formed and added onto the ribbon’s posterior end in the radular sac. The pattern and number of teeth in arrow are specific for each species are used in the classification of mollusc.

Foot

The molluscan foot may be variously adapted for locomotion, for attachment to substratum, or for combination of functions. It is usually ventral, sole-like structure in which waves of muscular contraction effect a creeping locomotion. However, there are many modifications, such as the following;

Attachment disc of limpets Laterally compressed “hatchet foot” of bivalves Siphon for jet propulsion in squid and octopus Wing-like parapodia or thin, mobile fins for swimming for pelagic (free-swimming) forms

Secreted mucus is often used as an aid of adhesion or a slime tract by small molluscs that glide on cilia. In snails and bivalves the foot is extended from the body hydraulically, by engorgement with blood.

Visceral MassIt depends on ciliary tracts for its functioning.

Mantle and Mantle CavityThe mantle is a sheath of skin, extending from the visceral mass that hangs down on each side

of the body, protecting the soft parts and creating between itself and the visceral mass a space called mantle cavity. Although the mantle is a major molluscan characteristic its role varies substantially in different molluscan groups just like the foot. The outer surface of the mantle secretes the shell.

The mantle cavity plays an enormous role in the life of a mollusc. It usually houses respiratory organs (gills or lungs), which develop from the mantle, and the mantle’s own exposed surface serves also gaseous exchange. The comb-like molluscan gills, known as ctenidia (ctenidia= G: comb). And also generally serves as the exit site for the excretory, digestive, and reproductive systems. A ctenidium (the singular form of “ctenidia”), when present, may have purely respiratory function or may also function in the collection and sorting of food particles. A chemoreceptor/tactile receptor known as the osphradium (osphra = G: a smell) is generally located adjacent to the ctenidium

ShellThe shell of mollusk, when present, is secreted by the mantle and is lined by it. Typically, there

are three layers. The periostracum is the outer organic layer, composed of an organic substance called

conchiolin, which consist of quinonetanned protein. It helps to protect underlying calcareous layers from erosion by boring organisms. It is a secreted by a fold of the mantle edge, and growth occurs only at the margin of the shell. On the older parts of the shell, periostracum often becomes worn away.

The middle prismatic layer is composed of densely packed prism of calcium carbonate (either aragonite or calcite) laid down in a protein matrix. It is secreted by the glandular margin of the mantle, and increase in shell size occurs at the shell margin as the animal grows.

The inner nacreous layer of the shell lies next to the mantle and is secreted continuously by the mantle surface, so that increases in thickness during the life of the animal life. The calcareous nacre is laid down in thin layers.

“Freshwater molluscs usually have thick periostracum that gives some protection against acids produced in the water by decay of leaf litter while marine molluscs’ periostracum is relatively thin and in some it is absent”

Internal Structure and Functions

Gas exchange occurs in specialized respiratory organs such as ctenidia, secondary gills and lungs, as well as the body surface, particularly the mantle. Most molluscs have open circulatory system with a pumping heart, blood vessels, and blood sinuses while cephalopods have a closed circulatory system with heart, vessels, and capillaries. Digestive tract is complex and highly specialized, according to feeding habits of the various molluscs and is usually equipped with extensive ciliary tracts.

Most molluscs have a pair of kidneys (metanephridia, a type of nephridium in which the inner end opens into the coelom by a nephrostome). Ducts of the kidneys in many forms also serve as for discharge of eggs and sperm.

The nervous system consists of several pairs of ganglia with connecting nerve cords, and it is generally simpler than the annelids and arthropods.

Reproduction and Life History

Most molluscs are dioecious, although some are hermaphroditic. The free-swimming trochophore larva that emerges from the egg in many molluscs is remarkably similar to annelids. Direct metamorphosis of a trochophore into a small juvenile, as in chitons, is viewed as ancestral for molluscs. However, in many molluscan groups (especially gastropods and bivalves) the trochophore stage is followed by a uniquely molluscan larval stage called a veliger. The free-swimming veliger has the beginnings of a foot, shell, and mantle. In many molluscs the trochophore stage occurs in the egg, and a veliger hatches to become the only free-swimming stage. Some molluscs especially cephalopods have no free-swimming larvae; instead juveniles hatch directly from eggs.

Trochophore larvae are minute, translucent, more or less pear-shaped, and have a prominent circlet of cilia (prototroch) and sometimes one or two accessory circlets. They are found in molluscs and annelids with primitive embryonic development and are usually considered homologous between the two phyla. Some form of trochophore-like larva is also found in marine turbellarians, nemertines, brachiopods, phoronids, sipunculids, and echiurids and together with recent molecular evidence, it suggests a phylogenetic grouping of these phyla. Based on developmental an molecular evidence, many zoologists unite them in a taxon called Trochozoa.

Mollusk Phylogeny

There are about 50,000 to 80,000 extant species and 40,000 extinct species. The first molluscs probably arose during Precambrian times. It is likely that molluscs split off from the line that led to annelids after coelom formation, but before segmentation appeared. “Hypothetical Ancestral Mollusc” have probably lacked a shell or crawling foot, small (about 1 mm), worm-like organism with a ventral gliding surface and probably possessed a dorsal mantle, a chitinous cuticle and calcareous scales.

Mollusk Taxonomy

Extant molluscs are distributed among 7 classes. Six of these 7 classes are represented by fossils formed some 450 million years ago, along with 1 additional class of molluscs, the Rostroconchia, whose clam-like members went extinct some 225 million years ago. All told, there are at least 35,000 molluscan species known only as fossils. Only one class of molluscs, the Aplacophora, has left no fossil record!Phylum Mollusca Classes1. Class Polyplacophora2. Class Aplacophora3. Class Monoplacophora4. Class Gastropoda5. Class Bivalvia

6. Class Cephalopoda7. Class Scaphopoda

Class Polyplacophora

Class Poly.placo.phora(G: many plate bearing)

Defining Characteristic: Shell formed as a series of 7 to 8 separate plates

The class has 800 species known as “chitons (G: coat of mail). Chitons are typically 3 cm to 10 cm (centimeters) long and generally found close to shore, particularly in intertidal zone; restricted to living on hard substrata, especially rocks. A chitons most distinctive external feature is its shell, which occurs as a series of overlapping and articulating plates (usually 8) covering the dorsal surface.

These plates are partially or largely embedded in the mantle tissue that secretes them. These plates overlap posteriorly and are usually dull colored to match the rocks to which chitons cling. Because the shell is multi-sectioned, the body can bent to conform to a wide variety of underlying substrate shapes. A chiton’s thick lateral mantle is called girdle. When disturbed, the chiton can press the girdle tightly against the substrate. This ability to cling tightly to the substrate is a particularly effective adaptation for life in areas of heavy wave action.

The chiton nervous system is simple and ladder-like. Ganglia are lacking in many species, and in others they are poorly developed. Their head and sensory organs are reduced but photosynthetic structures (aesthetes) which have the form of eyes in some chitons. Aesthetes-abundant organs derived from mantle tissues and extending through holes in the shell plates- are thought to function, at least in some species, as light receptor. Recent ultrastructural studies, however, suggests that aesthetes may function primarily in secreting periostracum, replacing material that is naturally abraded away in the highly turbulent environment in which most chiton live. The chiton mouth and anus are at opposite ends of the body, and the digestive tract is linear. Most chitons feed by projecting the radula outward from the mouth to scrape algae from the rocks, although the few species are carnivores. They move about and feed at night when they are less vulnerable to predators, such as birds

Even though most chitons ingest algae, a crystalline style is not a component of the digestive system; instead, a pair of pharyngeal glands, often called sugar glands, releasing amylase-containing secretions into the stomach

Chitons have a fossil record extending back some 500 million years ago. Sexes are separated in most chitons, and trochophore larvae metamorphose directly into juveniles, without an intervening veliger stage.

Class Aplacophora

Class A.placo.phora(G: not shell bearing)

Defining Characteristic: Cylindrical, vermiform body with the foot forming a narrow keel.

Aplacophorans are worm-shaped (vermiform) molluscs found in all oceans, mostly in deep water. Most species are quite small---- usually only a few millimeters and rarely more than a few centimeters. It is entirely marine. The body is unsegmented and bears numerous calcareous spines or scales embedded in an outer cuticle. The spines or scales are secreted by individual cells in the underlying epidermis; there is no true shell. Aplacophorans have no conspicuous foot, although the members of I group (the Solenogastres) possess nonmuscular, ciliated ridge located in a groove on the body’s ventral surface and believed to be homologous with the foot of other molluscs.

Two Subclasses1. Neomeniomorpha or Solenogastres2. Chaetodermomorpha or Caudofoveata

Subclass Solenogastres

The Solenogastres (less often referred to as Neomeniomorpha), common name solenogasters, are a subclass of worm-like, small, shell-less molluscs (Aplacophora), the other subclass being the Caudofoveata (Chaetodermomorpha).

In contrast to all other molluscan classes, the Aplacophora have no shell, and are instead covered by aragonitic sclerites (calcareous spicules), which can be solid or hollow. These spicules can be arranged perpendicular one another within the cuticle to form a skeleton; or can stick up to form a palisade; or can lay flat against the cuticle.

80% of solenogaster species have a radula, while in others it is secondarily lost. The radula may bear one or more teeth per row; where there is more than one tooth; there is no central radular tooth. The radula grows by dividing existing teeth in two, or by adding a new tooth at the centre of the radular row. The salivary glands are very elaborate, and are an important character for taxonomy. Next to the mouth they have a unique sense organ, the vestibulum.

The solenogastres do not have true ctenidia, although their gill-like structures resemble them. During development many solenogastres are covered by a spiny scleritome comprising spines or scale-like plates; this has been likened to the halwaxiid scleritome. Solenogasters feed on cnidaria and ctenophores, either sucking their bodily fluids or eating their tissue.

Subclass Caudofoveata

Caudofoveata are small (1-30 mm), mainly deep sea molluscs. They are worm-like, lacking shells or distinct muscular feet; they instead have scales and calcareous spines called sclerites, for movement.

They live by burrowing through soft sediment and feed by lying vertically in the sediment with just the mouthparts exposed and taking in passing organic detritus. During sexual reproduction, the female produces eggs which are fertilized and brooded, and then the larvae swim freely. Caudofoveates feed on foramenifera.

Class Monoplacophora

Class Mono.placo.phora

(G: one shell bearing)

Defining Characteristics: 1) 3 to 6 pairs of ctenidia 6 to 7 pairs of nephridia; 2) multiple (usually 8) pairs of foot (pedal) retractor muscles

Monoplacophora were long thought to be extinct until 1952 when living species of Neopilina (G: neo, new, + pilos, felt cap) were dredged up from the ocean bottom near the west coast of Costa Rica. All marine and all collected from depths of at least 2,000 m (meters). A single, unhinged, cap shaped shell is present, as in many limpets like gastropods. The shell of adult monoplacophorans is flattened rather than spirally wound, although the larval shell is spiral. Maximum adult shell lengths range from less than 1 mm in 1 species to about 37 mm in members of the largest species. The monoplacophoran foot is flattened, as in gastropods and polyplacophorans.

There are nearly 20 living species known. Cephalopods, scaphopods, gastropods, and bivalves—may have evolved from monoplacophorans ancestors. Unlike most other mollusks, monoplacophorans have some serially repeated organs. Such serial repetition occurs to a more limited extent in chitons. These animals have 3 to 6 pairs of gills, two pairs of auricles, three to seven metanephridia, one or two pairs of gonads, and a ladder-like nervous system with 10 pairs of pedal nerves.

Caudofoveata Solenogaster

Class Gastropoda

Class Gastro.poda

(G: Stomach foot)

Defining characteristics: 1) Visceral Mass and nervous system become twisted 90-180⁰ (exhibiting torsion) during embryonic development; 2) proteinaceous shield on the foot (operculum), to which columellar muscle attaches

It includes snails, limpets, slugs, whelks, conchs, periwinkles, sea slugs, sea hares and sea butterflies. The Gastropod is the largest molluscan classes, comprising 40,000 to 75,000 living species of snails and slugs distributed among marine, freshwater, and terrestrial environments. About 75% to 80% of all living molluscs are gastropods. Gastropods occupy very diverse habitats, including rivers, lakes, trees; deserts, the marine intertidal zone, the plankton, and the deep sea, and they exhibit a striking diversity of lifestyles, including suspension feeding, carnivorous, herbivorous, deposit-feeding, and ectoparasitic species. The typical snail consists of a visceral mass sitting atop a muscular foot. The visceral mass is commonly protected by a univalved shell that is typically coiled probably as an adaptation for efficient packaging of visceral mass.

The animal is attached to the inside of its shell by a columellar muscle, which extends from within the animal’s foot to the central axis of the shell; this central axis is known as columella. The columellar muscle is important in most major body movements: protraction from the shell, retraction into the shell, twisting, raising the shell above the substratum, and lowering it back down.

The shell is typically carried so that it leans to the left side of the body. The shell axis is thus oblique to the long axis of the body, balancing the animal’s center of mass over the foot. The shells of most gastropod species coiled clockwise, to the right and that is the shells are “right-handed” or “dextral” (dextro= L, the right hand side). Some species are “left-handed”, or sinistral in their coiling (sinister= L, the left hand side).

Many gastropod species possess, in addition to their shells, fairly elaborate behavioral or chemical defenses against predators. These adaptations commonly take 1 of the following forms:

1. The gastropods senses the presence of potential predators, either chemically or by touch, and initiates appropriate escape, avoidance, or deterrent behavior;

2. The gastropod chemically senses the presence of injured individuals of its own species (i.e., conspecific individuals) and initiates appropriate escape behavior; or

3. The gastropod accumulates noxious organic compound in its tissues, thereby becoming distasteful to potential predators.

TorsionThese animals are basically symmetrical, but because of torsion, a twisting process that

occurs in the veliger stage, their visceral mass has become asymmetrical. It is an anticlockwise twisting of most of the body (the visceral mass) through 180⁰ during early development.

Only gastropods undergo torsion. Torsion is usually a two-step process that occurs during veliger stage. The first step is usually rapid, and may take only a few minutes in some species. During this step, an asymmetrical foot retractor muscle contracts and pulls the shell and enclosed viscera 90⁰ counterclockwise to the head. The second step is much slower, and takes place over the remainder of larval development as a result of differential tissue growth. Before the torsion occurs, the embryo’s mouth is anterior and the anus and mantle cavity are posterior. By the end of the second stage of the torsion, the viscera have been pushed an additional 90⁰ counterclockwise, leading to the figure eight arrangement of the adult visceral nerves.

As a consequence of torsion, the nervous and digestive systems become obviously twisted, and the mantle cavity moves from the rear of the animal to become positioned over the head. After torsion, the anus and mantle cavity become anterior and open above the mouth and head. The left gill, kidney, and heart auricle are now on the left, whereas the original right gill, kidney, and heart auricle are now on the left, and the nerve cords have been twisted into a figure eight. Because of the space available in the mantle cavity, the animal’s sensitive head end can now be withdrawn into the protection of the shell, with the tougher foot, and when present the operculum, forming a barrier to the outside.

Varying degrees of detorsion are seen in opisthobranchs and pulmonates, and the anus opens to the right side or even to the posterior. However, both of these groups were derived from torted ancestors.

The curious arrangement that results from the torsion poses a serious sanitation problem by creating the possibility of wastes being washed back over the gills (fouling) and causes scientists to wonder what strong evolutionary pressures selected for such a strange realignment of body structures.

Coiling

Coiling, or spiral winding, of the shells and visceral mass is not the same as torsion. Coiling may occur in the larval stage at the same time as torsion, but the fossil records shows that coiling was a separate evolutionary event and originated in gastropods earlier than torsion did.

Early gastropods had a bilaterally symmetrical planospiral shell, in which all whorls lay in a single plane. Such shell was not very compact, since each whorl had to lie completely outside the preceding one. The compactness problem of a planospiral shell was solved by the conispiral shape, in which succeeding whorl is at the side of the preceding one. However, this shape was clearly unbalanced, hanging as it was with much weight over to one side. Better weight distribution was achieved by shifting the shell upward and posteriorly, with the shell axis oblique to the longitudinal axis of the foot. The weight and bulk of the main body whorl, the largest whorl of the shell, pressed on the right side of the mantle cavity, however, and apparently interfered with the organs on that side. Accordingly, the gill, auricle, and kidney of

the right side have been lost in most living gastropods, leading to a condition of bilateral asymmetry. Feeding Habits

Feeding habits of gastropods are as varied as their shapes and habitats, but all include use of some adaptation of the radula. Most gastropods are herbivores, rasping off particles of algae from hard surfaces. Some are grazers, some are browsers and some are planktonic feeders. Haliotis, the abalone holds seaweed with its foot and breaks off pieces with its radula.

Some snails are scavengers living on dead and decaying flesh; others are carnivores that tear their prey with radular teeth. Melongena feeds on clams, especially Tagelus, the razor clam, thrusting its proboscis between the gaping shell valves.

Members of genus Conus feed on fish, worms and molluscs. Their radula is highly modified for prey capture. A gland charges the radular teeth with highly toxic venom. Some of its species can deliver very painful stings, and in several species the sting is lethal to humans. The venom consist of peptides know as conotoxins that are specific for the neuroreceptors of its preferred preys. Conotoxins have become valuable tools in research on the various receptors and ion channels of nerve cells.

Cyphoma gibbosum and related species live and feed on the gorgonians in shallow, tropical coral reefs. These snails are commonly known as “flamingo tongues”. During normal activity, their brightly colored mantle enveloped the shell, but it can be quickly withdrawn to the shell when the animal is disturbed.

Some sessile gastropods, such as some limpets, are ciliary feeders that use gill cilia to draw in particular matter roll it into a mucous ball, and carry it to their mouth.

Internal Form and FunctionRespiration in most gastropods is carried out by a ctenidium located in the mantle

cavity. Pulmonates have a highly vascular area in their mantle that serves as a lung. Most of the mantle margin seals to the back of the animal, and the lung opens to the outside by a small opening called pneumostome. Many aquatic pulmonates must surface to expel a bubble gas from their lung. To inhale, they curl the edge of the mantle around the pneumostome to form a siphon.

Most gastropods have a single nephridium (kidney). The circulatory and nervous systems are well-developed. The latter incorporates 3 pairs of ganglia connected by nerves. A sensory area called an osphradium, located at the base of the incurrent siphon of most gastropods, is chemosensory in some forms, although its function may be mechanoreceptive in some and remains unknown in others.

There are both monoecious and dioecious gastropods. Many gastropods perform courtship ceremonies. During copulation in monoecious species there is an exchange of spermatozoa or spermatophores (bundles of sperms). Gastropods with the most primitive reproductive characteristics discharge ova and sperm into seawater where fertilization occurs, and embryos soon hatch as free-swimming trochophore larvae. In most gastropods’ fertilization is internal.

Gastropods play an important, although indirect, role in transmitting several major human diseases with many species serving as obligate intermediate hosts in the life cycle of

parasitic flatworms (Platyhelminthes). Indeed, much research on the control of these parasites has focused on regulation of the snail populations.

Three Subclasses:1. Prosobranchia2. Opisthobranchia3. Pulmonata

Subclass Prosobranchia (G: anterior gill)

Defining Characteristic: Mantle cavity generally anterior, due to torsion

Members of the Prosobranchia, largest of the three gastropod subclasses, are mostly marine, although a small percentage lives in freshwater or terrestrial environments. At least 20,000 species have been described. They are generally free-living and mobile, although some species may have evolved sessile or even parasitic lifestyles.

Prosobranchs are the most primitive of gastropods; that is, the other 2 group subclasses most likely evolved from prosobranch-like ancestors. Most gastropod species possess operculum, the rigid disc of protein (sometimes strengthen with calcium carbonate) in the foot.

The mantle cavity is anterior as the result of torsion, with the gill or gills lying in front of the heart. War enters the left side and exits from the right side, and the edge of the mantle often extends into along siphon to separate incurrent from excurrent flow. In prosobranchs with two gills (for example, the Haliotis and keyhole limpets Diodora), fouling is avoided by having the excurrent water go up and outthrough one or more holes in the shell above the mantle cavity.

Prosobranch have one pair of tentacles. Sexes are usually separate.

Subclass Opisthobranchia(G: posterior gill)Defining characteristic: Mantle cavity lateral or posterior due to detorsion, or lost

Members of the subclass Opisthobranchia, which includes sea hares, sea slugs, sea butterflies, and bubble shells. They are almost marine; most are shallow-water forms, hiding under stones and seaweeds; a few are pelagic. Fewer than 2,000 species have been described.The characteristic that distinguish this group from prosobranchs are

(1) A trend toward reduction or loss of the shell(2) Reduction or loss of the operculum(3) Limited torsion during embryogenesis(4) Reduction or loss of the mantle cavity(5) Reduction or loss of the ctenidia

Most species that have lost the ctenidia have evolved other respirator structures that are developmentally unrelated to the ancestral gill. For example, in many sea slugs (the

nudibranches—order Nudibranchia), gas exchange occurs across brightly colored dorsal projections called cerata, which also contain extensions of the digestive system.

Shell reduction or loss potentially increases vulnerability to predators, and it is reasonable to expect that pressures selecting for alternate means of defense have been quite strong. In particular, the cerata of many opisthobranch species house unfired defensive organelles (nematocyst) usurped from cnidarian prey. Instead of cerata, many other nudibranches possess feathery gills arising from the dorsal surface. Some species, including sea hares Aplysia, produce chemical defense against predation.

Opisthobranchs show partial or complete detorsion; thus the anus and gill (if present) are displaced to the right side or rear of the body. Clearly, the fouling problem is obviated if the anus is moved away from the head toward the posterior. Two pairs of tentacles are usually found, and the second pair, located dorsally, called rhinophores. The rhinophores are believed to be chemosensory, making them analogous to the osphradium of prosobranch gastropods.

The most primitive of the opisthobranchs are the bubble shells (order Cephalaspidea), which include the greatest number of opisthobranch species. These species show only little signs of detorsion. Although the locomotion is generally by means of cilia and pedal waves along the ventral surface of the foot, some opisthobranchs, such as the sea hares, can swim in short spurts by flapping lateral folds of the foot called parapodia. In other members of this subclass, the entire foot is drawn out into 2 thin lobes, also called parapodia, which are used for swimming. These animals are known as pteropods (“wing-footed”), or sea butterflies. Pteropods may or may not have shell, depending on the species, but all are permanents member of the plankton.

Subclass Pulmonata(L: lung)Defining Characteristic: Mantle cavity highly vascularized and otherwise modified to form a lung

In contrast to the prosobranch and opisthobranch gastropods, few of the 17,000 pulmonate species are marine, and hose few species occur only intertidally and in estuaries. Most pulmonate species are found in terrestrial or freshwater environments; slugs and “escargot” are terrestrial members of this subclass. A coiled shell is present in most pulmonates species, but the shell is reduced, internalized, or completely lost in others (the slugs). Torsion is limited to about 90⁰, so the nervous system is not so greatly twisted. They have lost their ancestral ctenidia, but their vascular mantle wall has become a lung, which fills with by contraction of the mantle floor. The anus and nephridiophore open near the pneumostome (pneumo = G: lung; stoma G: mouth)and waste is expelled forcibly with air or water from the lung

Class Bivalvia (= Pelecypoda)

Class Bi.valvia(L: two valved [hatchet foot])

Defining Characteristics: 1) Two-valved shell; 2) body flattened laterally

The class Bivalvia contains over 7,000 contemporary species, including clams, scallops, mussels, and oysters. Largely on the basis of gill structure, the bivalves may be divided into 2 major subclasses------the Protobranchia and the Lamellibranchia--- and 1 very small subclass, the Septibranchia. Major bivalve characteristics include

(1) A hinged shells, the two valves (left and right sides) of which are joined together by a springy ligament that springs the shell valves apart when the adductor muscles relax;

(2) Lateral compression of the body and foot;(3) Lack of cephalization: Virtual absence of a head and associated sensory structures;(4) A spacious mantle cavity, relative to that found in other molluscan classes;(5) A sedentary lifestyle; and(6) The absence of a radula/odontophore complex

Bivalves are primarily marine, but about 10% to 15% of all species occur in freshwater. No bivalves are terrestrial.

The hinged portion of a bivalve shell is dorsal. The shell valves, then, are on the animal’s left and right sides. The shell opens ventrally. A conspicuous bulge in the shell is frequently seen on the dorsal surface, adjacent to the hinge. This bulge, termed the umbo, is comprised of the earliest shell material deposited by the animal. Distinct growth lines typically run parallel to the shell’s outer margins. The foot projects ventrally and anteriorly, in the direction of movement, and the siphons, when present, project posteriorly.

Subclass Protobranchia(G: first gill)Defining Characteristics: 1) Gills small, functioning primarily as gas exchange surfaces; 2) food collected by long, thin, muscular extensions of surrounding the mouth (palp proboscides)

The group is entirely marine, and all species live in soft substrate. Most of the food collection in protobranchs bivalves is accomplished not by the gills, but rather by palp proboscides ---long, thin, muscular extensions of the tissue surrounding the mouth. The palp proboscides protrude between the shell valves and probe around the surrounding mud substrate, entangling particles in mucus. The sediment-laden mucus is then transported into the mantle cavity by cilia along the ventral surface of the proboscides. Attached to the bases of palp proboscides, within the mantle cavity, are flattened structures called labial palps; these sort the particles by ciliary action and to the margins of labial palps where these particles are ejected into the mantle cavity and expelled. This rejected material is termed as pseudofeces, since it is material that has never been ingested. Deposit feeding is their way of feeding.

Subclass Lamellibranchia(G; plate gill)Defining Characteristics: 1) Gills modified to collect suspended food particles, in addition to serving as gas exchange surfaces; 2) Secretion of proteinaceous attachment material (usually in the form of threads) by a specialized gland (the byssus gland) in the foot

Most bivalves are lamellibranchs. Although the majority of lamellibranchs are marine, all freshwater bivalve species are also members of this subclass.