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What are Mollusks? The name Mollusca (from the Latin mollis meaning soft), was first used by the French zoologist Cuvier in 1798 to describe squids. All mollusks have a muscular foot (used for moving) and a mantle (an outgrowth that covers the animal). Most mollusks have an external calcium carbonate shell that is produced by the mantle. All body systems are present. Many also have a radula (a unique organ that is mostly composed of a hard material called chitin) in the mouth that allows the animal to scrape food from surfaces by sliding back and forth. Like a rasping tongue. Mollusks have a coelom (a body cavity), but the coelom is made from cell masses, making all species in this phylum protosomes. All organs are suspended in this coelom, between the outer covering and the digestive tube of the animal. However, animals in this phylum are unique to coelomates in that they lack body segmentation. Taxonomy Body Systems of Mollusks Respiratory- diffusion still occurs (through mantle) but specialized gills, lungs present (derived from mantle). Circulatory- pumping heart, vessels and sinuses; open system in all but cephalopods; – Hemocyanin- oxygen combining pigment or “blood” that contains copper and is bluish when combined with oxygen. Digestive- complete and complex; mouth to anus. Excretory- 2 nephridia (kidneys) empty into mantle cavity. Nervous- ganglia and connecting nerve cord; specialized eye in cephalopods. Reproductive- most are dioecious (unisexual), but some gastropods are monoecious (heterosexual). (Go Back) Phylum: Molluska The 7 major classes belonging to Mulluska. Aplacophora - Mollusks without shells Bivalvia – Clams and their relatives Cephalopoda – Squids and their relatives Gastropoda - Snails and their relatives Monoplascophora - Mollusks with one plate Polyplacophora – Chitons Schapoda - Mollusks with a tubular shell (Go Back) Class: Aplacophora (belongs to Phylum: Molluska) This class includes two subclasses called Caudofoveata and Solenogastres. Both classes contain relatively few species. The Caudofoveata are aberrant molluscs which lack shells. They normally burrow in soft sediments and are quite common in the deep sea. The class Solenogastres is also very small. These animals also lack shells and generally live epibenthically (on the surface of the substratum). They usually resemble worms, and are found in deep water, often more than 3,000 m. There are only about 300 species in this class, and besides lacking shells, they are all marine and generally very small. NONE OF OUR SAMPLES BELONG TO APLACOPHORA. (Go Back) Chaetoderma canadense (an Aplascophoran) (Go Back) Class: Bivalvia (belongs to Phylum: Molluska) The bivalves are bilaterally symmetrical (can be divided along their length into two mirrored - right and left halves) with extensive mantle lobes which secrete a single shell composed of two valves. The two valves of the shell are hinged dorsally where they are held together by a new structure, the ligament, and completely enclose the rest of the body. The valves are closed by two large muscles. Bivalve respiration and locamotion reflect the morphology described. Class Bivalvia includes all of the bivalves. These are marine animals with a hinged shell divided into two halves. The hinge is head together by a ligament and one or two adductor muscles. Most live in sand or mud, and use their foot for digging and anchoring to surfaces. These bivalves use the hinges to take in food, and are also able to jet some distance away by closing the hinged shell and squirting the water taken in out of the mantle cavity. Bivalves lack a head and have a spacious mantle cavity as well. Some examples include clams, oysters, scallops, and mussels. The bivalves are mainly marine, but a few species are found in freshwater habitats, although none have invaded the land. Many species of bivalve are of commercial importance. NEXT Respiration in Bivalves The mantle cavity of bivalves is greatly expanded and now surrounds the rest of the body. Inside the mantle cavity are the large leaf-like ctenidia (feathery or comb-like structures), each of which consists of a dorsal (the back of an organ or body) vascular axis from which hang an inner and outer gill plate, or demibranch, made up of numerous parallel gill filaments. Each plate is not single, but is folded back on itself in a V-shape and thus has an ascending and a descending lamella. The two plates, when seen in cross section have a W-shape. Usually the lamellae are held together by junctions of different types; interlamellar junctions join the lamellae together and interfilamentar junctions join the adjacent filaments together. The more primitive bivalves, e.g. the mussel family, possess filibranch gills in which the interfilamentar junctions are only patches of interlocking cilia. (Go Back) Shape of lamellar plates (Go Back) Bivalve Locomotion Most bivalves are sedentary and many are adapted for burrowing into soft sediments using the laterally compressed foot. A few are capable of boring into wood, and some are even able to bore into rock. Although most bivalves are sedentary, some, especially members of the scallop family, are able to swim actively over short distances, usually to escape potential predators. The scallops swim by rapidly ejecting water from the mantle cavity by rapid clapping together (adduction) of the shell valves, which forces water out on either side of the hinge. Scallops usually swim in short bursts, but may cover several metres at a time. (Go Back) Orders belonging to Class: Bivalvia Major orders belonging to Mollusca/Bivalvia. Arcoida Myoida Mytiloida Pterioida Unionoida Veneroida (Go Back) Order: Arcoida (belongs to Class: Bivalvia) Major families belonging to Molluska/Bivalvia/Arcoida Arcidae (Ark shells) Glycymerididae (Bittersweet clams) (Go Back) Family: Arcidae (belongs to Order: Arcoida) These shells almost always have straight hinge lines and strong umbones. Most are ovate. Major Genera and species Anadara – brasiliana (Incongruous Ark) – transversa (Transverse Ark) Noetia – ponderosa (Ponderous Ark) (Go Back) Family: Glycymerididae (belongs to Order: Arcoida) These shells have V-shaped ligaments and many teeth. They are usually round or broadly ovate. Major Genera and species Glycymeris – pectinata (Comb Bittersweet) (Go Back) Order: Myoida (belongs to Class: Bivalvia) Major families belonging to Molluska/Bivalvia/Myoida Pholadidae (Piddocks) (Go Back) Family: Pholadidae (belongs to Order: Myoida) Major Genera and species Cyrtopleura – costada (Angel Wing) (Go Back) Order: Mytiloida (belongs to Class: Bivalvia) Major families belonging to Molluska/Bivalvia/Mytiloida Mytilidae (Mussels) Pinnidae (Pen shells) (Go Back) Family: Mytilidae (belongs to Order: Mytiloida) Major Genera and species Geukensia – demissa (Atlantic Ribbed Mussel) Ischadium – recurvum (Hooked Mussel) Modiolus – americanus (Tulip Mussel) (Go Back) Family: Pinnidae (belongs to Order: Mytiloida) Major Genera and species Altrina – rigida (Stiff Pen Shell) (Go Back) Order: Pterioida (belongs to Class: Bivalvia) Major families belonging to Molluska/Bivalvia/Pterioida Anomiidae (Jingle shells) Ostreidae (Oysters) Pectinidae (Scallops) Plicatulidae (Kitten’s Paws) (Go Back) Family: Anomiidae (belongs to Order: Pterioida) Major Genera and species Anomia – simplex (Common Jingle Shell) (Go Back) Family: Ostreidae (belongs to Order: Pterioida) Major Genera and species Crassostrea – virginica (Eastern Oyster) Ostrea – equestris (Crested Oyster) (Go Back) Family: Pectinidae (belongs to Order: Pterioida) Major Genera and species Argopecten – gibbus (Calico Scallop) (Go Back) Family: Plicatulidae (belongs to Order: Pterioida) Major Genera and species Plicatula – gibbosa (Kitten’s Paw) (Go Back) Order: Unionoida (belongs to Class: Bivalvia) Major families belonging to Molluska/Bivalvia/Unionoida NONE OF OUR SAMPLES BELONG TO Unionoida. (Go Back) Order: Veneroida (belongs to Class: Bivalvia) Major families belonging to Molluska/Bivalvia/Veneroida Cardiidae (Cockles) Carditidae (Carditas) Cultellidae (Razor clams) Donacidae (Donaxes) Lucinidae (Lucines) Mactridae (Surf clams) Petricolidae (Petricolas) Semelidae (Semeles) Solecurtidae (Razor clams) Solenidae (Razor clams) Tellinidae (Tellins) Veneridae (Venus clams) (Go Back) Family: Cardiidae (belongs to Order: Veneroida) Major Genera and species Dinocardium – robustum (Giant Atlantic Cockle) Trachycardium – egmontianum (Prickly Cockle) – muricatum (Yellow Cockle) (Go Back) Family: Carditidae (belongs to Order: Veneroida) Major Genera and species Carditamera – floridana (Broad-ribbed Cardita) (Go Back) Family: Cultellidae (belongs to Order: Veneroida) Major Genera and species Ensis – directus (Common Razor Clam) (Go Back) Family: Donacidae (belongs to Order: Veneroida) Major Genera and species Donax – variabilis (Florida Coquina) (Go Back) Family: Lucinidae (belongs to Order: Veneroida) Major Genera and species Divaricella – quadrisulcata (Crosshatched Lucine) Parvilucina – multilineata (Multi-lined Lucine) Phacoides – pectinatus (Thick Lucine) Pseudomiltha – floridana (Florida Lucine) (Go Back) Family: Mactridae (belongs to Order: Veneroida) Major Genera and species Mactra – fragilis (Fragile Atlantic Mactra) Mulinia – lateralis (Dwarf Surf Clam) Raeta – plicatella (Channeled Duck Clam) (Go Back) Family: Petricolidae (belongs to Order: Veneroida) Major Genera and species Petricola – pholadiformis (False Angel Wing) (Go Back) Family: Semelidae (belongs to Order: Veneroida) Major Genera and species Cumingia – tellinoides (Tellinlike Cumingia) (Go Back) Family: Solecurtidae (belongs to Order: Veneroida) Major Genera and species Tagelus – plebeius (Stout Tagelus) (Go Back) Family: Solenidae (belongs to Order: Veneroida) Major Genera and species Solen – viridis (Green Jackknife Clam) (Go Back) Family: Tellinidae (belongs to Order: Veneroida) Major Genera and species Tellina – – – – alternata (Alternate Tellin) iris (Iris Tellin) sybaritica (Dall’s Dwarf Tellin) tampaensis (Tampa Tellin) (Go Back) Family: Veneridae (belongs to Order: Veneroida) Major Genera and species Chione – cancellata (Cross-barred Venus) Dosinia – discus (Disk Dosinia) Macrocallista – numbosa (Sunray Venus) Merceneria – campechiensis (Southern Quahog) (Go Back) Class: Cephalopoda (belongs to Phylum: Molluska) This class contains the squids, octopuses and cuttlefish. Orientation of the body differs from most mollusks in that the ventral region (belly-side) is now anterior (up front) and the visceral mass is now the functional posterior (in back) end of the animal. This change in orientation has implications for sensory function and cephalopod locomotion. Also differs from other mollusks in the alteration of much of the original foot into a series of large prehensile tentacles or arms at the anterior end which now surround the head. Most either have an internal shell, like a squid. Some lack a shell, like an octopus. All cephalopods are predators, and use jaws and radula to crush and rip prey. Like bivalves, they are able to move by taking water into the mantle cavity and shooting it back out. They have large brains enclosed in cartilaginous brain cases. Because of a developed brain, good eye sight, and good sense organs, they are considered to be one of the most advanced invertebrates. NONE OF OUR SAMPLES BELONG TO CEPHALOPODA. (Go Back) Octopus (a Cephalopod) (Go Back) Eye of an Octopus (Go Back) Cephalopod Locomotion Most cephalopods, except the more sedentary octopus, are active animals, which swim by means of jet propulsion, during which water is rapidly forced out of the mantle cavity through the siphon by the contraction of the muscles in the mantle wall. Squids, besides being the fastest cephalopods, are also the fastest swimmers of all aquatic invertebrates. Their long tapered bodies are ideally streamlined, and the lateral fins provide stability. The 'flying squids' are particularly interesting. These have highly developed fin vanes and may shoot out of the water and glide for some distance. (Go Back) Cephalopod Tentacles Squids and cuttlefish have ten appendages comprised of eight arms and two long tentacles possessing numerous cup-shaped, rimmed suckers. The tentacles are used to capture prey, which is held by the tentacles and their suckers as it is pulled towards the mouth. The mouth lies at the base of the tentacles and possesses a parrot-like beak. The octopuses have eight long tentacles (sometimes called arms) each with a row of suckers on the inner surface. (Go Back) Class: Gastropoda (belongs to Phylum: Molluska) Gastropods are the most successful group of molluscs not only in terms of the number of species, but also in the wide range of habitat in which they may be found. Marine species have become adapted to living on all types of substratum and some have even adopted a pelagic (living in open water either at the surface or at intermediate depths) existence. Others have successfully invaded all types of freshwater habitat as well as the land. A univalve, generally spirally coiled shell is present in the majority of gastropods. The most significant change from the hypothetical ancestral mollusc that the gastropods have undergone during their phylogeny is the process of torsion. The third important feature of gastropods is the greater degree of head development or cephalization. Class Gastropoda is the largest group of mollusks, with between 40,000 and 75,000 species. These are also the only mollusks that live on land. Gastropods, like Class: Schapoda, lack gills, and use the mantle as a makeshift lung for extracting oxygen and breathing. Most species have coiled shells, and this class has evolved tentacles and eyes. This class includes marine animals like the sea slug and terrestrial animals like the slug and snail. NEXT Gastropod Shell Coiling One characteristic feature of the majority of gastropods is that the ancestral conical shell has now become coiled. This process was a separate evolutionary event and was not connected with torsion, indeed there is fossil evidence that it may have preceded it. In the most primitive gastropods, shell coiling was planospiral, whereas in the more advanced forms coiling is asymmetrical. In this type of shell, the shell axis has now become shifted slightly to improve weight distribution and in modern forms it is carried obliquely to the long axis of the body, and its weight is now borne by the widest part of the foot, near the middle of the body. The symmetrical shell and the angle at which it is carried has caused some occlusion of the mantle cavity on the right side, which in turn has resulted in the reduction or loss of the right ctenidium and the associated auricle. The shape of a gastropod's shell depends greatly on its habitat and its mode of life. (Go Back) Gastropod Torsion The most significant change from the hypothetical ancestral mollusc that the gastropods have undergone during their phylogeny is the process of torsion. During torsion, most of the body behind the head, including the mantle, mantle cavity and visceral mass are twisted anticlockwise through 180 degrees. Not all living gastropods are torted, however. Some groups have undergone detorsion during which the mantle cavity has moved to within 90 degrees of its original position, occasionally even 120 degrees in some species, and has resulted in some rearrangement of the internal anatomy. (GO BACK) Orders belonging to Class: Gastropoda 5 major orders belonging to Molluska/Gastropoda Archaeogastropoda Caenogastropoda Heterogastropoda Nudibranchia Sigmurethra (Go Back) Order: Archaeogastropoda (belongs to Class: Gastropoda) Major families belonging to Molluska/Gastropoda/Archaeogastropoda Turbinidae (Turban shells) (Go Back) Family: Turbinidae (belongs to Order: Archaeogastropoda) These shells are turban- or top-shaped, or occasionally flattened. They are thick, sculptured or smooth, with a rounded aperture that is pearly within. Major Genera and species Turbo – castanea (Chestnut Turban) (Go Back) Order: Caenogastropoda (belongs to Class: Gastropoda) Major families belonging to Molluska/Gastropoda/Caenogastropoda Conidae (Cone shells) Crepidulidae (Cup-and-saucer shells or slipper shells) Fasciolariidae (Spindle shells) Ficidae (Fig shells) Melongenidae (Crown conch shells) Muricidae (Murex shells) Nassariidae (Dog whelk or Basket whelk shells) Naticidae (Moon shells) Olividae (Olive shells) Terebridae (Auger shells) (Go Back) Family: Conidae (belongs to Order: Caenogastropoda) These shells are cone-shaped (narrow at bottom, wide at shoulder) with a low spire. Major Genera and species Conus – floridanus (Florida Cone) (Go Back) Family: Crepidulidae (belongs to Order: Caenogastropoda) These shells are either cap-shaped or slipper-shaped with a shelf underneath. Major Genera and species Crepidula – convexa (Convex Slipper Shell) – fornicata (Atlantic Slipper Shell) – plana (Eastern White Slipper Shell) (Go Back) Family: Fasciolariidae (belongs to Order: Caenogastropoda) These shells are usually large and are generally spindle-shaped. Major Genera and species Fasciolaria – tulipa (True Tulip) (Go Back) Family: Ficidae (belongs to Order: Caenogastropoda) These shells are thin and have the shape of a large fig or a slender pear. Major Genera and species Ficus – communis (Common Fig Shell) (Go Back) Family: Melongenidae (belongs to Order: Caenogastropoda) These shells are usually large with a large body whorl and a low to elongately conical spire. Major Genera and species Busycon – canaliculatum (Channeled Whelk) – carica (Knobbed Whelk) – contrarium (Lightning Whelk) (Go Back) Family: Muricidae (belongs to Order: Caenogastropoda) These shells usually have spines, but some have strong ribs instead. Major Genera and species Eupleura – caudata (Thick-lipped Drill) (Go Back) Thick-lipped Drill (Go Back) Family: Nassariidae (belongs to Order: Caenogastropoda) These are small snail shells that are usually elongate and conical. Major Genera and species Nassarius – obsoletus (Eastern Mud Whelk) – trivittatus (New England Basket Whelk) (Go Back) Family: Naticidae (belongs to Order: Caenogastropoda) These shells are round to broadly ovate. They are usually smooth with short spires. Major Genera and species Neverita – duplicata (Shark’s Eye) Sinum – perspectivum (Common Atlantic Baby’s Ear) (Go Back) Family: Olividae (belongs to Order: Caenogastropoda) These shells are elongate and cylindrical. They have a narrow aperture and a small conical spire. Major Genera and species Oliva – sayana (Lettered Olive) (Go Back) Lettered Olive (Go Back) Family: Terebridae (belongs to Order: Caenogastropoda) These shells are high, narrow, and elongate, with numerous whorls. The surface may be smooth or have spiral grooves. Major Genera and species Hastula – cinerea (Gray Atlantic Auger) Terebra – dislocata (Common American Auger) (Go Back) Common American Auger (Go Back) Order: Heterogastropoda (belongs to Class: Gastropoda) Major families belonging to Molluska/Gastropoda/Heterogastropoda Epitoniidae (Wentletrap shells) (Go Back) Family: Epitoniidae (belongs to Order: Heterogastropoda) Almost all of these shells are white with convex whorls and roundish apertures. Major Genera and species Epitonium – angulatum (Angulate Wentletrap) (Go Back) Angulate Wentletrap (Go Back) Order: Nudibranchia (belongs to Class: Gastropoda) Major families belonging to Molluska/Gastropoda/Nudibranchia NONE OF OUR SAMPLES BELONG TO Nudibranchia. (Go Back) Order: Sigmurethra (belongs to Class: Gastropoda) Major families belonging to Molluska/Gastropoda/Sigmurethra NONE OF OUR SAMPLES BELONG TO Sigmurethra. (Go Back) Class: Monoplascophora (belongs to Phylum: Molluska) This is a very small group of organisms that has been known for only a short time. Originally thought to exist only in the fossil record, they were rediscovered in 1952, and there are now about ten known species, all of which are deep ocean dwellers. This class is characterized by having an unhinged shell. The animals possess a large flattened foot surrounded by the pallial groove, in which are situated 5 or 6 pairs of gills. Internally, the animals possess 8 pairs of dorso-ventral pedal retractor muscles, a feature that is known to have existed in fossil forms. They also show a number of distinct characteristics such as the possession of 6 pairs of nephridia (excretory organ having function of kidney in invertebrates) lying in the pallial groove. The coelom consists of the pericardial cavity and the cavity of the two pairs of gonads. NONE OF OUR SAMPLES BELONG TO MONOPLASCOPHORA. (Go Back) Structure of Neopolina (a Monoplascophoran) (Go Back) Class: Polyplacophora (belongs to Phylum: Molluska) This class includes the chitons. The shells of chitons are characteristically divided into 8 transverse, overlapping shell plates or valves. The foot is greatly expanded, forming a large flattened sole which is used not only for locomotion but also for maintaining firm contact with the rock surface. Adhesion is effected mainly by the foot under normal conditions but, when the animal is disturbed, the girdle (the soft part surrounding the shell) is also clamped down tightly onto the rock surface. Chitons feed on small particles of algae on the rock surface which are scraped off using the radula. Feeding normally takes place when covered by the tide, and they tend to be most active when immersed at night. They are normally inactive animals and move only to feed, but often show 'homing behaviour', which enables them to return to the same area of rock. NONE OF OUR SAMPLES BELONG TO POLYPLACOPHORA. (Go Back) Chiton glaucus (a Polyplacophoran) (Go Back) Class: Schapoda (belongs to Phylum: Molluska) One of the smaller classes of molluscs, the scaphopods, or tusk shells, are burrowing, marine molluscs having a tubular tusk or tooth-shaped shell which is open at both ends. They are elongated along the anterior-posterior axis and live buried in the sand, head downwards, with the body steeply inclined. Water enters and leaves the mantle cavity via the posterior aperture. The head is very reduced and lacks eyes, but is surrounded by numerous thread-like tentacles or captacula. The captacula possess an adhesive knob at their tip and are used to gather small particles of food present in the sand and pass them to the mouth. Scaphopods are found mainly in the deep sea, but a few species occur in shallow water.This class is unusual because it lacks both gills and heart. Therefore, gas exchange occurs in the mantle and blood is circulated by contractions of the foot. NONE OF OUR SAMPLES BELONG TO SCHAPODA. (Go Back) Graptacme eborea (a Shapodan) (Go Back)