When the creature is disturbed or is at complete rest it may withdraw that portion of the mantle which protrudes from below the shell entirely within the edge of the shell, so that the "soft parts" may be wholly covered, and thus protected from harm. In such a state of rest nothing may be seen from above but a shell. The entire upper surface of the mantle is firmly attached to the under or inner side of the shell, except the mantle margin, this being free. The nervous system consists of a circumesophageal ring of nerve-cells, with two cords passing along the digestive tract to the posterior end of the animal. In the head and above the buccal mass are several series of nerve-ganglia. These also communicate with the posterior portion of the animal through two parallel cords which pass along its ventral side. From these four principal branches, which traverse the entire length of the creature, minor nerve-cords are thrown off to the tentacles, the foot, the branchiæ, and the various organs of the viscera. The sexes are separate. Reviewing our "typical mollusk," we find that it is bilaterally symmetrical—that is, uniform upon both sides of a median line from mouth to excretory opening; is unsegmented; has the ventral surface thickened into a creeping foot or disk; and is possessed of a head with tentacular processes, a radula, lateral branchiæ, a heart with a ventricle and two auricles, a well-developed mantle, and a shell. If the reader can learn these few details of organization he knows the structure of a mollusk. In turning to the actual living forms he will find merely modifications (though sometimes confusing ones) of these various parts. CLASSIFICATION The /Mollusca/ are divided into five great classes: /Amphineura/, /Gasteropoda/, /Scaphopoda/, /Pelecypoda/, and /Cephalopoda/. These names, it will be noticed, with the exception of the first, all refer to the /foot/, the modifications of the foot being selected as the basis of the first great divisions of the phylum. The /Gasteropoda/ crawl upon the thickened ventral surface of their bodies (already [pg321] referred to as the foot), as the common garden-snail, the periwinkles, and in general all those mollusks which have a spirally coiled shell. The /Scaphopoda/ have a long, worm-like foot, with which they burrow in the sand or mud. Their shells are like miniature elephant-tusks, but are open at both ends. The /Pelecypoda/ have a more or less club-shaped foot, utilized, in the many families, for a great variety of purposes. They are always inclosed in a bivalve shell. Familiar examples are the oysters, the clams, mussels, etc. The /Cephalopoda/ have the /foot/ modified into a number of /arms/, which encircle the head or the mouth. They are the cuttlefishes, the octopi, squids, etc. CLASS =AMPHINEURA= ORDER =POLYPLACOPHORA=