MOLLUSKS. 
LTHOUGH mollusks include creatures of most diverse forms 
- and habits, there are certain things which we may say of the 
race as a whole. 
In the first place their bodies are not divided into segments, 
as is often the case in worms and crustaceans. Moreover, the right 
and left halves of the body are typically similar each to each, 
although this is not the case in forms having coiled shells. The 
lower surface of the body consists of a thick muscular foot used 
in creeping. In front of the foot we find the head, which may 
have a pair of eyes and tentacles; while the mouth lies on its lower 
surface and is often provided with numerous horny, rasping teeth. 
A flap-like fold of the body extends outward from the sides. This 
fold is called the “mantle,” and its free edge and upper part 
secretes the shell which usually covers the back of the mollusk. 
The feathery gills arise from the sides, and he in the space between 
the lower side of the mantle and the side of the body. The intes- 
tine is coiled and opens typically at the posterior end of the body, 
behind the foot. There is a paired digestive gland or “liver” which 
pours its secretion into the mid-gut. The three-chambered heart 
lies above the hind gut and pumps blood from the gills to other 
parts of the body. The simplest mollusks are the Chitonide or 
armadillo slugs, specimens of which are commonly found crawling 
over heaps of dead shells. Their bodies are flat and oval in out- 
line, and the back is covered with eight shingle-like calcareous 
plates. The mouth is situated on the lower surface immediately 
in front of the broad muscular foot. A deep groove extends down 
each side of the body, and from the bottom of this arise the lancet- 
shaped gills. Chitons cling with great tenacity to the surface upon 
which they may be crawling, but if torn off they roll up into a ball. 
A common species with us, about half an inch long, is Trachy- 
dermon apiculata, which ranges from Cape Cod to the Gulf of 
Mexico. It is abundant upon dead oyster shells and is usually 
dull brown or gray in color, although some specimens are white. 
