• PHYLUM 6. MOLLUSCA, OR MOLLUSKS.
Snails, clams, squids, octopuses and related creatures. They are complex, unsegmented animals with three germ layers, a coelom and a digestive tube with mouth and anus. They have bilateral symmetry, although the snails are so twisted about in their coiled shells that they seem to have no symmetry. Mollusks have a layer of tissue around the body, called the mantle, which usually lay down a limy shell. Some mollusks, the squids, for example, have eyes, which operate very much as our own eyes do. Mollusks live mostly in the sea but are also numerous in fresh water and do pretty well on land. Many fossils have been found of long-extinct mollusks. About 80,000 species of living mollusks are known.