Molluscs (Mollusca) are invertebrates that include animals such as squid, octopuses, cuttlefish, nudibranchs, snails, slugs, limpets, sea hares, mussels, clams, oysters, scallops, as well as many lesser known creatures. There are an estimated 100,000 species of molluscs making them second largest phylum in the animal kingdom, having fewer species than only the Phylum Arthropoda.
1. There are eight groups of molluscs alive today.
Aplacophorans (Class Aplacophora) are marine molluscs that burrow into the sea floor in deep waters. They are small, wormlike creatures that lack shells or muscular feet. Aplacophorans grow to no more than two inches in length.
Bivalves (Class Bivalvia) are marine and freshwater molluscs that have a shell that consists of two valves that are hinged at the back. Bivalves have no head, their body consists of a wedge-shaped foot.
Caudofoveata (Class Caudofoveata) are small deep sea molluscs that burrow into soft bottom sediments. Like alpacophorans, they too lack shells and muscular feet characteristic of other molluscs. Caudofoveata have scale-like calcareous spicules.
Cephalopods (Class Cephalopoda) include octopus, squid, cuttlefish and nautilus. Cephalopods exhibit bilateral symmetry and in most members of the group the shell has either been internalized or lost. The exception is the nautilus which has retained a shell.
Gastropods (Class Gastropoda) include snails and slugs. Gastropods live in marine, freshwater, and terrestrial habitats. Gastropods is the most diverse class of mollusc with more than 60,000 species.
2. The body of most molluscs consists of two parts: a head-foot and a visceral mass.
In general, the head-foot of a mollusc is an elongated, soft structure that encompasses an anterior head and an elongated foot. The head has a pair of eyes, a mouth, sensory and nervous structures, and a radula. The foot is used for locomotion and for attaching onto surfaces. The visceral mass is located on top of the head-foot and contains internal organs that perform the functions of digestion, circulation, reproduction and excretion.
3. Molluscs have a mantle that secretes a hard shell that covers and protects the visceral mass.
A layer of tissue called the mantle covers the visceral mass of a mollusc. The mantle secretes a shell that encases the mantle and the soft visceral mass below it. The shell consists of three layers, an inner layer known as the nacreous layer, a middle layer called the prismatic layer, and an outer layer called the periostracum. The shell provides the mollusc with protection from predators and from harsh environmental conditions.
4. Molluscs exhibit bilateral symmetry.
5. Most molluscs have an open circulatory system.
6. Molluscs are more closely related to annelids than they are to arthropods.
7. There are approximately 100,000 species of molluscs.
Of all the molluscan subgroups, the most divers group is the gastropods. There are between 60,000 and 80,000 species of living gastropods. The bivalves are the second most diverse group of gastropods with about 30,000 species. There are about 800 species of cephalopods.
8. Some molluscs don't have a shell.
9. Some molluscs don't have a head.
10. The giant squid and the giant clam are among the world's largest molluscs.