Mollusca what are the major subclasses of phylum Mollusca?




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MOLLUSCA

What are the major subclasses of phylum Mollusca?
The classes Gastropoda, Bivalvia, and Cephalopoda each have diversified sufficiently that a brief r6sumt of their major subclasses is needed to complete our review of mollusks. Remember that each subclass possesses most of the characteristics of the class to which it belongs, so that only specializations and modifications will be considered here (Questions 13.,17-13.19).

What features distinguish the gastropod subclasses?
Gastropods are divided into three subclasses based on the presence and location of their gills (Fig. 13.1 1). A great variety of forms adapted for different modes of life occur among these groups.
(a) Members of subclass Prosobranchia ("forward gill") retain their larval torsion throughout life, so that the gill or gills and the anus remain in a forward position. This is true of the great majority of gastropods-manne, freshwater and terrestrial. Only one order, Archeogastropoda, including primitive forms such as limpets, keyhole limpets, abalones, tops, and turbans, has kept two gills, two auricles, and two nephridia. All other prosobranchs have only one gill, one auricle. and one nephridium. Order Mesogastropoda is characterized also by a radula with transverse rows of seven teeth; it includes littorines, turrets, wormshells, conchs, cowries, helmets, tuns, moon shells, violet sea snails, and heteropods. The last two deserve further mention, for both live in the open sea. Violet sea snails (Janthina) float beneath bubble rafts, preying especially upon the purple by-the-wind sailor Velella, a cnidarian (see Fig. 9.5a). Heteropods have a very reduced shell and a finlike foot by which they swim (see Fig. 13.6a). Order Neogastropoda is characterized by a radula with only three teeth per transverse row; it includes drills or murexes, whelks, volutes, olives, augers, and the venomous cones.
(b) Members of subclass Opisthobranchia ("rear gill") undergo detorsion, untwisting so that the anus and gill return to the posterior position; as a result of having undergone torsion as larvae, opisthobranchs have only one gill, one auricle, and one nephridium. Most opisthobranchs possess shells too small for their bodies to retreat into; some have only flimsy internalized shells, and others known as nudibranchs ("naked gill") have no shell at all, and their gills are exposed on the body surface, not tucked away under flaps of the mantle (see Fig. 13.6a). Species of the sea hare, Aplysia (so called for its rabbit-ear cephalic tentacles), may reach a chubby length of I m, thus being the largest known gastropods. Nudibranchs that feed on cnidarians are often brilliantly colored, warning predators that they can deliver secondhand stings by virtue of undischarged nematocysts stored in their integuments and in the plumose cerata that cover the backs of many. Pteropods ("wing foot"), or sea butterflies, are pelagic forms, with and without shells, that have armlike extensions (parapodia) used like wings to "fly" through the water (see Fig. 13.6a).
(c) Subclass Pulmonata includes terrestrial and aquatic gastropods that possess one auricle, one nephridium, and no gills at all. Instead, the mantle cavity is modified into a vascularized sac used for gaseous exchange on land and, secondarily, in water (see Fig. 13.6b). The intertidal onchids are slugs that have a posterior pulmonary sac and posterior anus. Pulmonates undergo torsion during development even though they pass through no trochophore or veliger larval stages, and most retain torsion throughout life, with anus and pulmonary opening located forward at the shell edge just behind the head. Those having one pair of cephalic tentacles are mostly freshwater snails, such as Lymnea. Pulmonates with two pairs of tentacles include terrestrial snails, such as Helix, and land slugs, such as Limax.

What are the subclasses of bivalves?
The class Bivalvia, or Pelecypoda, is subdivided taxonomically on rather technical grounds, especially the nature of the hinge teeth and gill structure.
(a) Protobranchs ("first gill") have gill filaments arranged in rows along either side of a central axis and not folded into a W shape. Their foot is flattened ventrally. They include nut clams (subclass Palaeotaxodonta), which have taxodont articulation (i.e., a row of short teeth along the hinge margin), and solemyids (subclass Cryptodonta), which have thin, elongate valves lacking hinge teeth.
(b) Lamellibranchs have enlarged gills, with filaments folded back into a W shape. Adjacent filaments are interconnected either by cilia (filibranchiate) or by fleshy connections (eulameUibranchiate). Members of subclass Pteriomorphia are epibenthonic (i.e., living on, not in, the bottom of a body of water), often attaching themselves by one valve (e.g., oysters) or by byssus threads (e.g., mussels). Pteriomorphs have filibranchiate gills and lack siphons; they include arks, which have taxodont hinging, and also mussels, pens, oysters, jingle shells, and scallops. Scallops are particularly talented bivalves, for they can jet either forward or backward by expelling water from between their mantle flaps and are guided by two entire rows of remarkably well developed eyes along the mantle edges.

Members of subclass Palaeoheterodonta are mostly freshwater bivalves; these have eulamellibranchiate gills, but their siphons are very short and not tubular.



The great majority of burrowing clams, with tubular siphons, eulamellibranchiate gills, and shells lacking a nacreous lining, belong to subclass Heterodonta, which includes cockles, chamids, razor clams, giant clams, rockboring pholadids, wood-boring shipworms, and several species of freshwater clams. Giant clams (Tridacna) at first bore into coral reefs, but as they grow, they eventually fall free to live epibenthonically, usually lying with valves agape, sunning the symbiotic algae that brilliantly color the much folded mantle. Giant clams may exceed a mass of 200 kg, and it is imprudent to thrust arm or leg between their gaping valves.
(c) Septibranchs (subclass Anomalodesmata) have one or no hinge teeth, and their gills are reduced to a muscular septum that divides the mantle cavity into dorsal and ventral chambers. These bivalves are predatory and employ pumping contractions of the septum to suck small worms and crustaceans into the mantle cavity, where they are seized by the muscular labial palps and shoved voraciously into the mouth.

What are the subclasses of class Cephalopoda?
(a) Subclass Nautiloidea is a group of great antiquity, seen in Cambrian strata but represented today only by the chambered nautilus (Nautilus) (see Fig. 13. 10b), which gives us some idea as to the body organization of the 2500 extinct forms of nautiloids. The chambered nautilus has two pairs of gills, two pairs of nephridia, and a large number (to more than 90) of tentacles that lack suckers. The eye is of the pinhole-camera type, lacking a lens for focusing, but instead having a small pupil through which seawater enters the eyeball. The siphon is not tubular but bilobed. An ink sac is lacking. Nautilus has a planicoiled external shell divided into chambers by septa, so that the globular body occupies only the newest and largest chamber. A cordlike ligament extends through the septa to the apex of the original chamber and anchors the nautilus within its shell, which it cannot leave. The gas-filled chambers serve as buoyancy regulators, allowing the nautiloid to hover at a given depth. They are not easy to observe alive in the wild, for by day they stay below the range of light penetration. rising to within scuba depth only at night. Extinct nautiloids probably bore close resemblance to the one surviving genus, but some had tapering, uncoiled shells.
(b) Subclass Ammonoidea is known only from fossil forms. The ammonites existed from the Silurian to the end of the Cretaceous, when they abruptly became extinct along with many other forms of marine and terrestrial animal life (see Chap. 4). That ammonites became extinct and nautiloids did not (quite) seems attributable to the fact that ammonite young lived in the surface plankton and were much tinier when hatched than nautiloid youngsters, which hatched at a larger size and lived in deeper water, where they would be better buffered from climatic crises. Ammonites had planicoiled, chambered, external shells that in some species grew to the size of a large tnick tire. The suture lines marking septal attachments were often elaborately convoluted, rather than simple as in nautiloids.
(c) Subclass Coleoidea includes all dibranchiate cephalopods, which have only one pair of gills and nephridia. They lack an external shell, but have an ink sac. The eyeball is of the closed vesicular type, with anterior and posterior fluid-filled chambers, comea, iris, and lens, and seems to focus images clearly albeit nearsightedly. The siphon is tubular, permitting swift jet propulsion and efficient maneuvering. The mantle is external and may bear lateral fins. The tentacles, or arms, number 8 or 10 and bear suckers. Coleoids include the extinct belemnites, known from their internal, straight, chambered shells. Living forms having eight shorter arms and two longer, extensible tentacles that are shot out to grab prey include: Spirula, with its internal, planicoiled, chambered shell; cuttlefish, with bladelike cuttlebone; and squid, with an internal horny pcn or plate and a usually elongated body bearing posterior fins. The vampire squid are small, deep-sea fon-ns that have two tiny retractile tentacles and eight large arms united in umbrella fashion by an interbrachial web reaching almost to the tips, allowing them to swim like a medusa and capture prey within the web. The octopods have only eight arms and a globular body lacking fins. Although most are benthonic, including the familiar octopus, the argonaut uses modified paddle-shaped arms to propel itself in the open sea. The female carries along, and can retreat into, her delicate, planicoiled egg case, for which she is often called a "paper nautilus," even though she is not a nautilus at all. The male often remains dwarfed and rides within his mate's egg case.


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