The Molluscs of this class are characterized by bilateral symmetry. The head and arms are situated at the opposite extremities of the elongated body, the gills, genital ducts and circulatory organs being paired and similar on both sides. The first order belonging to this division, the Polyplacophora, includes the “Coat-of-mail shells,” or “Sea-woodlice” (_Chitonidæ_). They have their back armed with eight shelly plates which overlap one another like tiles, and, like woodlice, have the power of rolling themselves into a ball. These plates are imbedded at the sides into the fleshy mantle, beneath which, on each side of the foot, are arranged the gills. A Chiton differs in many respects from other Mollusca. It has a shell like an Isopod Crustacean, a heart down the back like a sea-worm, symmetrical organs of reproduction on each side like the bivalves, a head and crawling foot like a true Limpet, and a posterior anal orifice. These several anatomical peculiarities at one time induced certain eminent authorities to hesitate in considering them molluscs; but now that the development from the egg has been investigated, their association with the Mollusca may be considered definitely settled. [Illustration: Fig. 2. Coat-of-mail Shells, or Chitons. 1. _Chiton squamosus_ (upper surface). 2. _Chiton elegans_ (lower surface): _a_, mouth; _b_, foot; _c_, mantle; _d_, gills. ] Chitons are found in all parts of the world, the finest inhabiting tropical countries. They live chiefly on rocks and under stones at low-water or at moderate depths; but a few forms have been discovered by the ‘Challenger’ Expedition at depths exceeding 2000 fathoms. The numerous sections of the group are principally distinguished by differences in the edges of the plates or valves which are inserted in the mantle, and in the different kinds of ornamentation upon the upper surface of the mantle-border. This, in some species, is quite smooth, in others covered with a dense mass of minute grains or scales, and in others armed with short prickly spines. In the giant _Cryptochiton_ of Kamtschatka the plates are entirely covered over by the thick leathery granular mantle, and in another set, _Cryptoplax_, which consists of long slug-like animals, the plates are very small, and placed at intervals along the back. About five hundred living species are known, and about one-fourth that number has been found fossil from the Silurian age downwards. [Sidenote: Case 3.] The second order of Amphineura, namely, the Aplacophora, comprises a few somewhat worm-like Molluscs which are devoid of a shell, but have instead the dorsal surface more or less studded with numerous minute calcareous spines or spicules. _Neomenia_, _Proneomenia_, _Chætoderma_ are genera belonging to this order. Class II.—GASTROPODA. [Sidenote: Cases 4–135.]