The digestive tract, beginning at the mouth, opens immediately into a muscular throat, which is called the /buccal mass/. Within the buccal mass there is a long, fleshy ribbon that bears upon its upper surface numerous rows of sharp teeth; these rows of teeth run crossways on the ribbon, each row containing more or less prominent median teeth, with several laterals upon each side, and with marginal teeth on each side of the laterals. This is called the /radula/, and is used for tearing or rasping the food. When [pg319] not in use it lies coiled up within the buccal mass. Beyond the buccal mass, food passes into the esophagus, which widens out into a muscular pouch or stomach; this may also be provided with chitinous teeth or hard knobs or plates for the purpose of further triturating the food. Above and partially surrounding the stomach is a large reddish or brownish liver, which furnishes to the alimentary canal the digestive juices. From the stomach the intestine leads to the terminal anal opening. There are two organs, serving the function of kidneys, placed dorsally one upon each side of the body (/nephridia/). A heart lies within the median line of the body, and consists of a ventricle and two lateral auricles. The vascular system is almost entirely closed; only in so far as it is partially open does it essentially differ from that of higher groups; the blood is whitish or light bluish, and receives its oxygenation in the gills or along the free mantle surface, through which it courses in exceedingly thin capillaries. In this manner respiration is carried on to some extent throughout the whole outer surface of the body. There are two /branchiæ/ (gills). These have the appearance of small feathers and are placed upon each side of the animal posteriorly; they project slightly. Spread over the top of this animal, fitting it perfectly, is a thin fleshy covering attached to its dorsal portion like a skin, but hanging free, like a flap, over the sides, the head, and the posterior end of the animal. Upon the sides it may just cover the tips of the branchiæ, but in front the head is seen to project from under it. The loose-hanging portion of this sac-like covering is somewhat thickened, and perhaps recurved upward upon itself. Upon the edge of this flap all the way around may occur a row of cilia, or tentacular processes. This cloak-like covering is called the mantle, and, as already noted, is the most peculiar and persistent characteristic of the /Mollusca/. The "cavity" formed between the loose-hanging portion of the mantle and the sides of the foot is called the /mantle cavity/, and contains posteriorly the gills, or branchiæ. From the outer surface of this mantle a calcareous secretion takes place, forming a shell, which, in our ideal mollusk, will be shaped in conformity with the surface outlines of the mantle, oblong and convex above. Over the lower portion of the shell the edge of [pg320] the mantle recurves.