Phylum Mollusca By: Charmaigne Molina Phylum Mollusca is the second largest of all animal phyla; it includes between 50, 000and 110,000 living species, depending upon who is doing the counting. It came from the Greekword 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, makingthe 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 carbonateset in a protein matrix. Organic material may comprise about 35% of the shell’s dry weight insome gastropod species and up to about 70% of the dry weight in bivalves. This very diverse group includes chitons, tusk shells, snails, slugs, nudibranches, seabutterflies, clams, mussels, oysters, squids, octopuses, and nautiluses. The group ranges fromfairly simple organisms to some of the most complex of invertebrates; sizes range from almostmicroscopic 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)Habitat
Molluscs are found in a great range of habitats, from the tropics to polar seas, at altitudesexceeding 7000 m, in ponds, lakes, and streams, on mud flats, in pounding surf, and in openocean from the surface to abyssal depths. They represent a various lifestyles, including bottomfeeders, burrowers, borers, pelagic forms. According to fossil evidence, mollusc originated in the sea, and most of them haveremained there. Much of their evolution occurred along the shores, where food was abundantand habitats were varied. Only bivalves and gastropods moved into brackish and freshwaterhabitats. As filter feeders, bivalves were unable to leave aquatic surroundings. Only slugs andsnails (gastropods) actually invaded the land. Terrestrial snails are limited in their range by theirneed 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 areformed 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 andthe shells’ inner surface, a pearl may form over a period of years. Natural pearl formation is a fairly rareevent; 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 (usuallyfrom freshwater bivalves) or plastic spheres between the shell and mantle of mature oysters, and thenkeeping the oysters alive and protected for 5 to 7 years. Note that cultured pearl form in a perfectlynormal 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 theravages of shipworms, wharves must be creosote or built of concrete (unfortunately, some shipwormsignore creosote, and some bivalves bore into concrete). Snails and slugs frequently damage the garden and other vegetation. In addition, snails oftenserve as intermediate hosts for serious parasites of humans and domestic animals. Boring snails of genusUrosalpinx 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-footportion 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 oncilia. In snails and bivalves the foot is extended from the body hydraulically, by engorgement with blood.Visceral Mass It depends on ciliary tracts for its functioning.
Mantle and Mantle Cavity The mantle is a sheath of skin, extending from the visceral mass that hangs down on eachside of the body, protecting the soft parts and creating between itself and the visceral mass a space calledmantle cavity. Although the mantle is a major molluscan characteristic its role varies substantially indifferent 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 housesrespiratory organs (gills or lungs), which develop from the mantle, and the mantle’s own exposed surfaceserves also gaseous exchange. The comb-like molluscan gills, known as ctenidia (ctenidia= G: comb). Andalso 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 functionin 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 ctenidiumShell The 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 packedprism of calcium carbonate (either aragonite or calcite) laid down in a protein matrix. It is secreted by theglandular 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 continuouslyby the mantle surface, so that increases in thickness during the life of the animal life. The calcareous nacreis laid down in thin layers.
“Freshwater molluscs usually have thick periostracum that gives some protection againstacids produced in the water by decay of leaf litter while marine molluscs’ periostracum is relatively thin andin some it is absent”Internal Structure and Functions Gas exchange occurs in specialized respiratory organs such as ctenidia, secondary gills andlungs, as well as the body surface, particularly the mantle. Most molluscs have open circulatory system witha pumping heart, blood vessels, and blood sinuses while cephalopods have a closed circulatory system withheart, vessels, and capillaries. Digestive tract is complex and highly specialized, according to feeding habitsof 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 theinner end opens into the coelom by a nephrostome). Ducts of the kidneys in many forms also serve as fordischarge of eggs and sperm. The nervous system consists of several pairs of ganglia with connecting nerve cords, and itis generally simpler than the annelids and arthropods.Reproduction and Life History Most molluscs are dioecious, although some are hermaphroditic. The free-swimmingtrochophore larva that emerges from the egg in many molluscs is remarkably similar to annelids. Directmetamorphosis 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 followedby a uniquely molluscan larval stage called a veliger. The free-swimming veliger has the beginnings of afoot, shell, and mantle. In many molluscs the trochophore stage occurs in the egg, and a veliger hatches tobecome the only free-swimming stage. Some molluscs especially cephalopods have no free-swimminglarvae; instead juveniles hatch directly from eggs. Trochophore larvae are minute, translucent, more or less pear-shaped, and have a prominentcirclet of cilia (prototroch) and sometimes one or two accessory circlets. They are found in molluscs andannelids with primitive embryonic development and are usually considered homologous between the twophyla. 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 aphylogenetic grouping of these phyla. Based on developmental an molecular evidence, many zoologistsunite 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 areseparated in most chitons, and trochophore larvae metamorphose directly into juveniles,without an intervening veliger stage. Class AplacophoraClass 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 indeep water. Most species are quite small---- usually only a few millimeters and rarely morethan a few centimeters. It is entirely marine. The body is unsegmented and bears numerouscalcareous spines or scales embedded in an outer cuticle. The spines or scales are secreted byindividual cells in the underlying epidermis; there is no true shell. Aplacophorans have noconspicuous 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 behomologous with the foot of other molluscs.Two Subclasses1. Neomeniomorpha or Solenogastres2. Chaetodermomorpha or Caudofoveata
Subclass Solenogastres The Solenogastres (less often referred to as Neomeniomorpha), commonname solenogasters, are a subclass of worm-like, small, shell-less molluscs (Aplacophora), theother subclass being the Caudofoveata (Chaetodermomorpha). In contrast to all other molluscan classes, the Aplacophora have no shell, and areinstead 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. Theradula may bear one or more teeth per row; where there is more than one tooth; there is nocentral radular tooth. The radula grows by dividing existing teeth in two, or by adding a newtooth at the centre of the radular row. The salivary glands are very elaborate, and are animportant character for taxonomy. Next to the mouth they have a unique sense organ, thevestibulum. The solenogastres do not have true ctenidia, although their gill-like structures resemblethem. During development many solenogastres are covered by a spiny scleritome comprisingspines or scale-like plates; this has been likened to the halwaxiid scleritome. Solenogasters feedon cnidaria and ctenophores, either sucking their bodily fluids or eating their tissue.Subclass Caudofoveata Caudofoveata Solenogaster Caudofoveata are small (1-30 mm), mainly deepsea molluscs. They are worm-like, lacking shells ordistinct muscular feet; they instead have scales andcalcareous spines called sclerites, for movement. They live by burrowing through soft sedimentand feed by lying vertically in the sediment with just themouthparts exposed and taking in passing organicdetritus. During sexual reproduction, the femaleproduces eggs which are fertilized and brooded, andthen the larvae swim freely. Caudofoveates feedon foramenifera. Class MonoplacophoraClass Mono.placo.phora
(G: one shell bearing)Defining Characteristics: 1) 3 to 6 pairs of ctenidia 6 to 7 pairs of nephridia; 2) multiple (usually8) pairs of foot (pedal) retractor muscles Monoplacophora were long thought to be extinct until 1952 when living species ofNeopilina (G: neo, new, + pilos, felt cap) were dredged up from the ocean bottom near the westcoast of Costa Rica. All marine and all collected from depths of at least 2,000 m (meters). Asingle, unhinged, cap shaped shell is present, as in many limpets like gastropods. The shell ofadult monoplacophorans is flattened rather than spirally wound, although the larval shell isspiral. Maximum adult shell lengths range from less than 1 mm in 1 species to about 37 mm inmembers of the largest species. The monoplacophoran foot is flattened, as in gastropods andpolyplacophorans. There are nearly 20 living species known. Cephalopods, scaphopods, gastropods, andbivalves—may have evolved from monoplacophorans ancestors. Unlike most other mollusks,monoplacophorans have some serially repeated organs. Such serial repetition occurs to a morelimited extent in chitons. These animals have 3 to 6 pairs of gills, two pairs of auricles, three toseven metanephridia, one or two pairs of gonads, and a ladder-like nervous system with 10pairs of pedal nerves. Class GastropodaClass 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 andsea butterflies. The Gastropod is the largest molluscan classes, comprising 40,000 to 75,000living species of snails and slugs distributed among marine, freshwater, and terrestrialenvironments. About 75% to 80% of all living molluscs are gastropods. Gastropods occupy verydiverse 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 consistsof a visceral mass sitting atop a muscular foot. The visceral mass is commonly protected by aunivalved shell that is typically coiled probably as an adaptation for efficient packaging ofvisceral mass. The animal is attached to the inside of its shell by a columellar muscle, which extendsfrom within the animal’s foot to the central axis of the shell; this central axis is known ascolumella. The columellar muscle is important in most major body movements: protractionfrom the shell, retraction into the shell, twisting, raising the shell above the substratum, andlowering it back down. The shell is typically carried so that it leans to the left side of the body. The shell axis isthus 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”, orsinistral in their coiling (sinister= L, the left hand side). Many gastropod species possess, in addition to their shells, fairly elaborate behavioralor chemical defenses against predators. These adaptations commonly take 1 of the followingforms: 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.Torsion These 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 areposterior. By the end of the second stage of the torsion, the viscera have been pushed anadditional 90⁰ counterclockwise, leading to the figure eight arrangement of the adult visceralnerves. As a consequence of torsion, the nervous and digestive systems become obviouslytwisted, and the mantle cavity moves from the rear of the animal to become positioned overthe head. After torsion, the anus and mantle cavity become anterior and open above themouth and head. The left gill, kidney, and heart auricle are now on the left, whereas theoriginal right gill, kidney, and heart auricle are now on the left, and the nerve cords have beentwisted into a figure eight. Because of the space available in the mantle cavity, the animal’ssensitive head end can now be withdrawn into the protection of the shell, with the tougherfoot, and when present the operculum, forming a barrier to the outside. Varying degrees of detorsion are seen in opisthobranchs and pulmonates, and the anusopens to the right side or even to the posterior. However, both of these groups were derivedfrom torted ancestors. The curious arrangement that results from the torsion poses a serious sanitationproblem by creating the possibility of wastes being washed back over the gills (fouling) andcauses scientists to wonder what strong evolutionary pressures selected for such a strangerealignment 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 showsthat coiling was a separate evolutionary event and originated in gastropods earlier thantorsion did. Early gastropods had a bilaterally symmetrical planospiral shell, in which all whorls layin a single plane. Such shell was not very compact, since each whorl had to lie completelyoutside the preceding one. The compactness problem of a planospiral shell was solved by theconispiral shape, in which succeeding whorl is at the side of the preceding one. However, thisshape was clearly unbalanced, hanging as it was with much weight over to one side. Betterweight distribution was achieved by shifting the shell upward and posteriorly, with the shellaxis 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, andapparently interfered with the organs on that side. Accordingly, the gill, auricle, and kidney ofthe right side have been lost in most living gastropods, leading to a condition of bilateralasymmetry.
Feeding Habits Feeding habits of gastropods are as varied as their shapes and habitats, but all includeuse of some adaptation of the radula. Most gastropods are herbivores, rasping off particles ofalgae from hard surfaces. Some are grazers, some are browsers and some are planktonicfeeders. 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 carnivoresthat tear their prey with radular teeth. Melongena feeds on clams, especially Tagelus, therazor clam, thrusting its proboscis between the gaping shell valves. Members of genus Conus feed on fish, worms and molluscs. Their radula is highlymodified for prey capture. A gland charges the radular teeth with highly toxic venom. Some ofits 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 ofits preferred preys. Conotoxins have become valuable tools in research on the variousreceptors 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 normalactivity, their brightly colored mantle enveloped the shell, but it can be quickly withdrawn tothe shell when the animal is disturbed. Some sessile gastropods, such as some limpets, are ciliary feeders that use gill cilia todraw in particular matter roll it into a mucous ball, and carry it to their mouth.Internal Form and Function Respiration in most gastropods is carried out by a ctenidium located in the mantlecavity. Pulmonates have a highly vascular area in their mantle that serves as a lung. Most ofthe mantle margin seals to the back of the animal, and the lung opens to the outside by a smallopening called pneumostome. Many aquatic pulmonates must surface to expel a bubble gasfrom their lung. To inhale, they curl the edge of the mantle around the pneumostome to forma siphon. Most gastropods have a single nephridium (kidney). The circulatory and nervoussystems are well-developed. The latter incorporates 3 pairs of ganglia connected by nerves. Asensory area called an osphradium, located at the base of the incurrent siphon of mostgastropods, is chemosensory in some forms, although its function may be mechanoreceptivein some and remains unknown in others. There are both monoecious and dioecious gastropods. Many gastropods performcourtship 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. Prosobranchia 2. Opisthobranchia 3. PulmonataSubclass Prosobranchia(G: anterior gill)Defining Characteristic: Mantle cavity generally anterior, due to torsion Members of the Prosobranchia, largest of the three gastropod subclasses, are mostlymarine, although a small percentage lives in freshwater or terrestrial environments. At least20,000 species have been described. They are generally free-living and mobile, although somespecies may have evolved sessile or even parasitic lifestyles. Prosobranchs are the most primitive of gastropods; that is, the other 2 group subclassesmost likely evolved from prosobranch-like ancestors. Most gastropod species possessoperculum, 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 ofthe heart. War enters the left side and exits from the right side, and the edge of the mantleoften extends into along siphon to separate incurrent from excurrent flow. In prosobranchswith two gills (for example, the Haliotis and keyhole limpets Diodora), fouling is avoided byhaving the excurrent water go up and outthrough one or more holes in the shell above themantle 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, seabutterflies, and bubble shells. They are almost marine; most are shallow-water forms, hidingunder 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 thatare developmentally unrelated to the ancestral gill. For example, in many sea slugs (thenudibranches—order Nudibranchia), gas exchange occurs across brightly colored dorsalprojections called cerata, which also contain extensions of the digestive system. Shell reduction or loss potentially increases vulnerability to predators, and it isreasonable to expect that pressures selecting for alternate means of defense have been quitestrong. In particular, the cerata of many opisthobranch species house unfired defensiveorganelles (nematocyst) usurped from cnidarian prey. Instead of cerata, many othernudibranches possess feathery gills arising from the dorsal surface. Some species, including seahares 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 theanus is moved away from the head toward the posterior. Two pairs of tentacles are usuallyfound, and the second pair, located dorsally, called rhinophores. The rhinophores are believedto 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 littlesigns 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 inshort spurts by flapping lateral folds of the foot called parapodia. In other members of thissubclass, the entire foot is drawn out into 2 thin lobes, also called parapodia, which are usedfor 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 permanentsmember of the plankton.Subclass Pulmonata(L: lung)Defining Characteristic: Mantle cavity highly vascularized and otherwise modified to form alung In contrast to the prosobranch and opisthobranch gastropods, few of the 17,000pulmonate 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 pulmonatesspecies, but the shell is reduced, internalized, or completely lost in others (the slugs). Torsion islimited to about 90⁰, so the nervous system is not so greatly twisted. They have lost theirancestral ctenidia, but their vascular mantle wall has become a lung, which fills with bycontraction 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 thelung 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 2major 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’sleft and right sides. The shell opens ventrally. A conspicuous bulge in the shell is frequently seenon the dorsal surface, adjacent to the hinge. This bulge, termed the umbo, is comprised of theearliest shell material deposited by the animal. Distinct growth lines typically run parallel to theshell’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) foodcollected 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 foodcollection in protobranchs bivalves is accomplished not by the gills, but rather by palpproboscides ---long, thin, muscular extensions of the tissue surrounding the mouth. The palpproboscides protrude between the shell valves and probe around the surrounding mudsubstrate, entangling particles in mucus. The sediment-laden mucus is then transported intothe 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; thesesort the particles by ciliary action and to the margins of labial palps where these particles areejected 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 toserving as gas exchange surfaces; 2) Secretion of proteinaceous attachment material (usually inthe form of threads) by a specialized gland (the byssus gland) in the foot Most bivalves are lamellibranchs. Although the majority of lamellibranchs are marine, allfreshwater bivalve species are also members of this subclass.