- Molluscs (Phylum Mollusca)
- The molluscs, or soft-bodied animals
- The molluscs are also invertebrate
- The molluscs having (80,000 species)
- The molluscs include oysters, mussels, slugs, snails, squids and cuttlefishes.
Body form of Molluscs
We can divide molluscs into four regions;
- A dorsal visceral mass which is coiled in the gastropods and in which the internal organs are concentrated in a rounded mass
- Head with tentacles and eyes (this is not in the bivalves)
- A ventral and muscular foot, a flat ‘sole’ on which the mollusc crawled, the foot can be prominently modified but is rarely lost
- A mantle consisting of a sheet of tissue that develops over the visceral mass and secretes a calcareous shell laid down as crystals within an organic matrix.
The shell of molluscs yields many forms;
it is normally in one piece (even in the bivalves where the two lateral valves and the dorsal connecting ligament together form the shell),
although in the polyplacophorans or chitons it consists of eight plates. The shell may become enclosed in the mantle and so reduced as in the slugs, some chitons, and the squids.
Consideration of the basic form in molluscs reveals that, unlike that of any other phylum, it has two growth axes: the antero-posterior axis of the head-foot such as occurs in all bilaterally sym- metrical animals, and-what must have appeared when the earliest mollusc evolved a second dorso- ventral axis. This is associated with the visceral mass and mantle or visceropallium. Growth in this instance takes place radially by marginal increment around the edge of the mantle where new shell is added. Many complications of molluscan form can only be understood by reference to the interacting symmetries of the bilateral head-foot and the radial (or biradial) visceropallium. They may achieve a balance or one may dominate the other.
While the ventrally placed muscular foot with its primitively broad ‘sole’ is responsible for temporary attachment to a hard substrate, it is also the prime means of locomotion. It is attached to the dorsal shell by a series of shell muscles, paired in all but the gastropods where, following torsion (see below), they are reduced to a single enlarged footlar mule Extension of the foox is achieved by hydratic pressure and withdrawal by mcular contraction Contractions of intimi ile within the foot produce the thythmic prope wave which canies the animal stewly on the fo can be extrmely meelitied to permis moment through sal or other soli cubrates, as in typical bivalves, or as ‘wings or fine for twimming in surface dwelling gastropods to a few cases where the animal becomes permanently attached the tot is lost Rapid movement in the cephalopods and in the bivalve scallops is produced by je propulsion when water is expelled from the mantle cavity by sudden muscular contractions. In the cephalopods the direction of the jer is controlled by the foot, converted into a tubular funnel.