The Mollusca are one of the largest groups of marine organisms and four of the five mollusc classes are represented in the North Sea. Living molluscs occur in terrestrial, freshwater and marine ecosystems, and in the latter case have become adapted to every kind of habitat from abyssal oozes to oceanic surface currents.
Molluscs are unsegmented, bilaterally symmetrical animals. The body consists of a muscular foot, a variously developed head, and a soft, non-muscular visceral mass. The foot may be adapted for grasping the substratum, for locomotion, burrowing or feeding, and is often closely associated with the head. In the gastropods the head is usually well developed, with paired sensory organs and often specialised feeding apparatus. The buccal cavity contains a complex, eversible structure, the odontophore, associated with a chitinous, toothed ribbon, the radula. In bivalves the head has regressed, and feeding and sensory functions are carried out by other parts of the body. In cephalopods the head and foot are integrated in the anterior, tentacle-bearing end of the animal. The main body organs of the mollusc are concentrated in the visceral mass, primitively situated dorsally towards the posterior end of the animal. The visceral mass is enclosed by a wide fold of body wall, the mantle or pallium. The mantle edge secretes the calcareous external shell, characteristic of molluscs, although secondarily absent in many taxonomic groups. The shell is three-layered, with an organic outer layer, the periostracum, a middle layer of columnar calcite, and an inner layer of laminated calcite, often nacreous. The mantle skirt covers the whole of the animal and typically incorporates a mantle or pallial cavity, primitively posterior in position. In primitive gastropods it contains the anus, the paired openings of the gonads and kidneys, and paired gills, or ctenidia, together with sensory organs, osphradia, and a mucus-secreting hypobranchial gland. Circulation and sanitation within the pallial cavity are aided by cilia and mucus-secreting cells. In the bivalves, in which the whole animal lies within an enlarged mantle cavity, ciliary/mucus gill cleansing has been adapted to microphagous feeding; the head has largely disappeared, and the mouth is equipped with pronounced labial palps for the collection of detritus-laden mucus.
In all molluscs the coelom is represented by a small cavity surrounding the heart and gonads. The blood circulatory system is open, and both blood and coelomic fluid circulate through extensive haemocoelic spaces, which serve as an efficient hydrostatic skeleton.
In primitive molluscs sexes are separate, fertilisation is external, and embryos develop as planktonic trochophore larvae. Although most marine molluscs are still dioecious, hermaphroditism is the rule in terrestrial groups; simultaneous or consecutive hermaphrodites are known among certain families of marine molluscs, while the subclass Opisthobranchia is entirely monoecious.
[After Hayward et al., 1990]