Clams, octopuses, squids, snails, slugs, mussels and oysters are examples of mollusks. Mollusks are invertebrates that have a head, a visceral mass and a muscular lower body part called a foot. Some have hard external shells and some do not.
The seven classes of mollusks are Monoplacophora, Aplacophora, Gastropoda, Polyplacophora, Bivalvia, Scaphapoda and Cephalopoda. Limpets and abalones are part of the class Gastropoda. Chitons look similar to pill bugs, have eight outer plates and are part of the class Polyplacophora. Clams, oysters, mussels and scallops are part of the class Bivalvia, of which 80 percent are found in the ocean. Cuttlefish, octopods and nautiluses are part of the class Cephalopoda. This class has the most well-developed nervous system and eye of all the mollusks. Mollusks lack body segmentation, but they do have a nervous system, a circulatory system and a respiratory system. Some mollusks are extraordinarily large, such as the 20-meter-long giant squid, and others are smaller than grains of sand, such as the aplacophorans.
Mollusks are found on mountaintops and in the hot vents and cold depths of the deep sea. Gastropods and bivalves can be found in the hot vents in the deep sea. Although over 50,000 species of mollusks are known, some species are on the verge of extinction, such as the white abalone.