This sub-kingdom, to which oysters, snails, cuttle-fish, etc., belong, includes unsegmented animals devoid of internal skeleton, and with bilaterally symmetrical (p. 16) embryo, while the adult may be much modified, especially in snails, which are enclosed in spiral shells, and also have their bodies partially coiled. The skin of molluscs possesses a peculiar and characteristic covering. From a definite part of the body a larger or smaller fold of skin (“mantle”) grows out, which encloses a space termed the “mantle-cavity,” and also, from its function, the “respiratory cavity.” In most molluscs this mantle secretes an external or internal calcareous mass. In this way a “shell” is developed (Fig. 143, A, _S_), which is usually external. This is the case, for example, with the edible, or Roman snail, and the common garden snail. In other cases (cuttlefishes), the shell is formed inside the mantle, or it may be replaced by a collection of calcareous granules (various slugs). In most molluscs the chief organ of locomotion is the so-called “foot” (Fig. 143, _F_). This is a very muscular region of the body, covered by skin, and serving either for crawling (snails), or else for digging in the sand, and even springing (bivalve molluscs). Its structure varies according to its function. I mention three classes, those of _Cephalopoda_ (Cuttlefishes), _Gastropoda_ (Snails and Slugs), and _Lamellibranchiata_ (Bivalve Molluscs). [Illustration: FIG. 143.—Diagrams to explain the form of the body of a Mollusc. A, a Bivalve Mollusc; B, a Snail (cross sections). In both; _K_, body; _F_, foot; _S_, shell; _m_, mantle; _k_, _k_, gills. ] CLASS: =CEPHALOPODA= (CUTTLEFISHES). The head is sharply marked off from the rest of the body, and the mouth is surrounded by a circle of arms bearing suckers or claw-like hooks. These arms are supposed to be a modification of part of the foot, the rest of this organ being constituted by a hollow, funnel-shaped structure, which places the mantle-cavity in communication with the exterior. The water used for respiration is forced suddenly out from the mantle-cavity, through the funnel, so as to propel the animal in the opposite direction. A gland, known as the “ink sac,” secretes a brownish fluid, which may be pressed out of the sac so as to shield the animal from attack. All cephalopods are marine, and, since they are therefore of no agricultural importance, it is unnecessary to enter into details regarding their form and structure, and I will only mention the following: =Pearly Nautilus= (_Nautilus pompilius_), with external shell and numerous arms; =Paper Nautilus= (_Argonauta argo_), with a very thin, boat-shaped shell, and eight arms; =Poulpe= (_Octopus_), without shell, and with eight arms; =Cuttle-fish= (_Sepia_), and =Squid= (_Loligo_), also without external shell, and possessing ten arms. CLASS: =GASTROPODA= (SNAILS AND SLUGS).