Molluscs include some of the best known invertebrates, with animals such as snails, clams, crabs, lobsters, squids, etc. The large phylum Mollusca contains more than 93,000 species already described and it is though that almost the same amount of species exist but still remain unknown because of their habitats, difficult to access. The name “Mollusca” comes from the Latin word “molluscus”, which means thin-shelled, and was attributed by Linnaeus in 1650. However, this first classification was very general and was grouping all the soft-bodied animals together. It is only in 1795 that the first approximate modern classification was made by Cuvier.
The molluscs are divided in seven very distinctive classes, and although we might think that the different individuals seem to differ greatly, they all share certain key characteristics.
Molluscs are bilaterally symmetrical, unsegmented and they possess a coelom (fluid cavity) that is limited to small spaces around the heart and parts of the intestine. The principal body cavity is called the hemocoel and is a blood-filled cavity. A thick epidermal sheet of skin called the mantle covers their body and during development the mantle undergoes folds that will create a specific space called the mantle cavity. The mantle cavity houses the gills, receives the products from the excretory and reproductive systems and houses shells glands. These shell glands will produce an interesting structure that all molluscs possess, with the exception of the class Aplacophora, which is a calcareous shell. The shell can vary greatly in shape and size and shows an amazing spectrum of colors and motifs.
Molluscs have a complete gut specialized with marked regions, like the digestive ceca.
The body wall of molluscs comprises three different layers: cuticle, epidermis and muscle. Regarding their locomotion, most molluscs possess a well-defined foot, with the exception of the aplacophorans and the chaetodermomorphans. The majority of the molluscs move mainly by waves of muscular contractions that pass along the foot.
They can greatly vary in shape and size. The microscopic bivalves are in the range of mm whereas the giant squid can reach an overall length of 20m.
Among molluscs, there are two different types of feeding: herbivory or predation. Molluscs possess a foraging structure called “radula”, which is used by both herbivores and predators and is a feature unique to molluscs. It consists of a ribbon of recurved chitinous teeth and serves as a fighter to remove food particles and cutting and chewing the food before it goes into the esophagus. The radula helps to smooth the digestion.
As mentioned earlier, there are 7 classes of Mollusca: Aplacophora, Monoplacophora, Polyplacophora (chitons), Gastropoda (snails), Bivalvia (clams), Scaphopoda (tusk shells) and Cephalopoda (squid, octopuses).