I. _Pelecypoda_, the Mussels--mollusks inclosed in a bivalve shell fastened by a muscular hinge, the adjacent part of the valves being generally more or less toothed; the foot is as a rule roughly comparable to the shape of an ax head. II. _Amphineura_, the Chitons--flattened, bisymmetrical mollusks whose shell consists of eight crosswise, overlapping plates. III. _Gastropoda_, Snails, whelks, etc.--mollusks that crawl on the flat undersurface of the body, or distensible foot. IV. _Scaphopoda_, Tusk shells--mollusks that possess a long tubular shell open at both ends; with their small and elongated foot they are supposed to _dig_ into the mud in which they live. V. _Cephalopoda_, Cuttlefishes, and Octopods--mollusks with tentaclelike "arms" arranged about the mouth, and either an external or internal shell. These are the highest in rank. THE OYSTER AND ITS RELATIVES The lowest in rank of these classes is the Pelecypoda, containing the "bivalves"--mussels, clams, oysters, and the like, in which the shell is in two parts or valves hinged together over the "back" of the animal, and attached to it on each side by a powerful muscle, the "adductor," by the contraction of which the shell may be tightly shut. Within the shell the body is enveloped in a "mantle," or fleshy membrane falling like a cloak on each side; and from it is secreted the outer shell, which grows by additions to its ventral margin. These additions are in a general way annual, so that the concentric lines of growth on its exterior are an indication of the years of the mollusk's life, which is slow in growth, and long-lived. The interior of the shell is usually pearly, and marked with microscopic rugosities, which, by breaking up the light, as if by innumerable prisms, gives the iridescence so beautiful in the pearl oyster, the fresh-water unios and many others. These pearly layers are called "nacre." Bivalves were formerly classified in conchology as Acephala, because they have no proper head, but at the posterior end are two openings of tubes, provided with cilia. In one, the cilia induce a constant current of water which after leaving the gills brings into the animal's stomach floating microscopic food, both plants and animals, including eggs and larvæ, where it is captured and assimilated while water is ejected through the other (dorsal) pipe. This food includes bacteria, and if the mollusk lives and feeds in water polluted by sewage, or otherwise containing germs of disease, it becomes dangerous as human food; hence oysters and clams exposed to such bad conditions ought never to be sent to market because of the disease germs remaining in them.