Main Mollusc Classes
These are worm-like molluscs, without any prominent dorsal visceral mass. Their ventral side is often imperfectly shaped in foot. For many of them, all around the body, the mantle is shaped in a curve into what numerous gills are regularly placed. The two coelomic cavities keep their primitive aspect and communicate with each other. The nervous system is analog to the one of worms, but doubled ventrally. The shell, when it exists, is formed of distinct calcareous plates.
Ex.: the Chitons that live along European coasts and creep on the rocks. In deep waters, often on hydra colonies live small Amphineura, the Neomenians, which have no shell.
Gastropoda have been submitted to simulatenous flexions and torsions that have determined their structure. The Gastropod is submitted to a flexion that is proportionnal to its shortening; a dorsal bulge lifts and the intestin describes a U shaped curve, this is the endogastric flexion. The dorsal bulge lifts and as it becomes longer it rolls up like a screw, the spire point is turned forwards. The anus remains in normal position, to the opposite of the mouth. At this moment, the Gastropod has carried out the exogastric roll up.
The third phase encompasses a true torsion. The intestinal mass turns onto itself, carrying out a 180° rotation. After this movement, the palleal cavity and its annexes pass in front, and the shell point is then turned to the rear. The torsion has another consequence: it causes the atrophy, more or less pronounced, of the intestins that are morphologically situated to the right and pressed by the left intestins that became right; thus, a single kidney subsists. The torsion occurs very quickly in the larva. During this torsion, the nervous system follows the movement and is shaped in eight (8). Such a system is called streptoneura.
The Gastropoda class is sub-divided in three sub-classes: Prosobranchia, Opisthobranchia and Pulmonata. Here are some definitions that enable to sort it all out.
The nervous system is streptoneura and sexes are separated. Their coelomic cavities do not communicate any longer, the genital organ is independent from the kidney coeloma; but the nephridia still communicates with the heart. Most of them are marine: Patelle, Murex, etc ...In freshwater, Prosobranchia also have numerous representatives, for example the Viviparous Paludine. Some of them are terrestrial such as the Cyclostoma; their gills are atrophic, they breathe like pulmonata. A series of Prosobranchia become swimmers and live in deep sea, these are the Heteropoda. their volumous and laterally flattened foot takes the shape of a ship keel; their long body sometimes has the shape of fishes. their body, rich in water, becomes transparent. this is the case of Carinaria, Pterotrachea, frequently caught with coastal plancton.
These are all marine and belong to another evolution line. The cardiac ventricle is turned forwards or to the side, whereas the gill is oriented to the rear or to the side. For a long time they were mistakenly considered as deformed Prosobranchia, it seems that for Opisthobranchia, the torsion - given the shape of their shell - is never more than 90°. The anus remains far behind the head, to the right side. the nervous system presents an incomplete torsion; often vey concentrated, it escapes any torsion. the intestinal mass is less developped than Prosobranchias, it has a tendency to blend with the rest of the body and stresses the symmetrical aspect of the animal. Opisthobranchia are hermaphrodite. some examples are the Doris taht creep on marine algae, the Aeolis that have no shell, Aplysias or sea hares with a partly internal shell. Pteropoda are planctonic Opisthobranchia.
They usually have no gilled palleal cavity. The upper part of the palleal cavity is welded to the body, leaving only a narrow hole or pneumostome. This hollow palleal cavity becomes the lung; its walls are irrigated by numerous vessels that gather in one major lung vessel that leads to the heart. The lung becomes the breathig organ and replaces a missing gill. Pulmonata have suffered a complete torsion as Prosobranchia, but their nervous system, generally very concentrated in the cephalic area, escapes any torsion. the position of the heart is similar to the one of Prosobranchia. Like Opisthobranchia, from which they drastically differ, they are hermaphrodite. Many live in freshwater: Limnaea, Bulinus, Planorbis; others are terrestrial: snails, slugs.
Whereas these are the most complex and evolved molluscs, they easily return to the primitive type. They developped in heigth without loosing their bilateral symetry. They pass through an endogastric flexion that is followed by a rolling up of the intestinal mass, more or less apparent according to the familly, but they do not pass through any torsion phase. Their shell varies considerably. The Nautilus shell rolls up in spire in one plan; transversal walls divide it into successive chambers what number indicates the age of the animal that lived successively in each one and lives in the most recent and bigger one. For example, in the cuttlefish the shell is reduced to an internal blade; in the squid, this shell is reduced to a hollow tube and the octopus has no shell.
These Molluscs are the most distant from the primitive type. They have a bilateral symmetry. Their foot is flattened and resembles the iron part of an axe (explaining the naim of Pelecypoda they are sometimes given) or a tongue; it is mostly used to dig. the intestinal mass makes no dorsal bulge and the cephalic area, extremely small (Acephala), is only represented by the mouth as seen from the exterior. No trace of mouth bulb remains and the radula has disappeared.
For example, the Nucula, the Mossels (Mytilus) that fix on rocks by their byssus, a set of strings similar to silk that is generated by a gland situated at the base of the foot; Oysters (Ostrea, Gryphea); Solens; Pholada, that dig holes in rocks. In freshwater, Unios and Anodonta, that live dug in the silt and what development encompasses a parasitic phase: their larva or glochidium is fixed to the gills or skin of fishes with hooks and feeds through a pseudo-pallium (false-mantle) that functions as an absorbing device.
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