Mollusks are invertebrates that live in ocean, freshwater and terrestrial habitats. It is estimated that there are as many as 120,000 different species of mollusk. There are several different classes of mollusk. The most common mollusks are the gastropods (e.g., snails and slugs), the cephalopod (e.g., octopus, squid and cuttlefish) and the bivalve (e.g., clam, muscle, oyster and scallop). The Giant Squid and the Colossal Squid are the largest animals without a backbone.
Gastropods make up 80% of the mollusk species with 60,000 to 80,000 species. They are the second most populous animals after the insects. Some gastropods, such as the snail, have an external shell large enough for the soft parts to withdraw completely inside the shell. Gastropods without a shell, or with a very small external shell, are known as slugs. Gastropods usually have a well-defined head with two or four sensory tentacles with eyes. They also have a foot, that they use to crawl along the ground or sea floor.
There are about 800 species of cephalopod, and are found only in oceans of the world. None exist in freshwater or on land. Cephalopods have a large head and a series of arms or tentacles. For example, the cuttlefish and squid have five pairs of tentacles surrounding their mouths. Two are longer tentacles used to capture prey. The Cuttlefish and squid have five pairs of muscular appendages surrounding their mouths. The longer two, termed tentacles, are actively involved in capturing prey. The octopus has four pairs of arms (i.e., 8 arms) with a mouth and hard beak at the center of the arms. The cephalopod’s primary means of movement, although somewhat slow, is using its arms to crawl or swim. For fast movement, the cephalopod expels water from a cavity in their body, similar to jet propulsion. Most cephalopods have an ink sac, and can expel a cloud of ink to help escape from predators.
There are about 30,000 species of bivalves including clams, muscles, oysters and scallops. Bivalves have a shell consisting of two halves, called valves, joined together at one edge by a flexible ligament called the hinge. Bivalve shells vary greatly in shape; some are quite rounded, while others are more flattened.