MOLLUSRS 319 



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 auri- 

 cles. 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 branchial (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 bran- 

 chine, 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 charac- 

 teristic of the Mottusca. 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 branchiae. 

 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 



