The phylum mollusca is characterized more by diversity than uniformity, though modern molluscs do all tend to share certain features, including the presence of a radula (a tonguelike ribbon of teeth), the structure of the nervous system and the presence of the mantle, which contains the animal's internal organs. Other than these few common characteristics, molluscs can be widely different and, according to the University of California, Berkeley, the phylum mollusca is one of the most diverse phylums known to man. Even the radula is not a completely uniform characteristic; though most members of the phylum mollusca have one, the bivalvia class does not.
The phylum mollusca is divided into seven classes, including bivalvia: aplacophora, polyplacophora, monoplacophora, gastropoda, cephalopoda and scaphopoda. Molluscs in several classes have a characteristic, distinct head and foot region, though this is not true of all mollusc classes, including cephalopoda, in which the 'foot' has been evolutionarily modified into arms such as can be seen in octopi and squid. Members of this phylum range from the microscopic members of the class aplacophora to the massive giant squid, a member of class cephalopoda. Many members of different classes within phylum mollusca have been used, or had parts used, by humans as sources of food, tools and jewelry.