There are more than
100,000 described species of Mollusc alive today and 35 000 fossil species, they range in size from just 2 mm to over
20 m in the giant squid, and occupy all major habitats.
They are believed to have originated in a Precambrian marine environment where they occupied the shore area offering them a great abundance and diversity of food and habitats. It is these varied conditions which probably led to their great diversity.
Later bivalves and gastropods moved from salt to brackish then fresh water habitats. And eventually some gastropods left the water and invaded the land, but were limited by their need for humidity and calcium.
Some molluscs have
multiple pairs of organs, e.g. Monoplacophora, and so were thought to have
evolved from a segmented ancestor making them closely related to the Annelids. This is no longer thought to be the case.
It is now believed that the molluscan ancestor was probably more like a flatworm that developed some sort of
protective calcareous covering over its dorsal surface. This covering would
have limited respiration, and it is thought that the evolution of gills may
have occurred at the same time. Two extinct phyla, Hyolitha and Wiwaxiida have
protective coverings that may be similar to the ancestral molluscan shell.
molluscs are slow-movers. Movement is by muscular ripples down a flat, sticky
pad. However the cephalopods (octopus, squid) move by a form of jet propulsion
by squirting water out from their mantle cavity.
The mollusc shell
The shell is usually formed
of three distinct layers of different forms of calcium carbonate, from the
outside the first layer is the periostracum, then a thick prismatic layer, and finally layers of nacre.
The mantle forms a
skirt, and retractor muscles can pull the shell down over the foot when the
animal is disturbed. The mantle cavity usually holds the gills or lungs, and the products from the digestive, excretory and reproductive systems are usually emptied into the mantle cavity.