MOLLUSKS 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 branchie (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- 
chi, 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 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 branchie. 
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 
