MOLLUSKS. 



\ LTHOUGH moUusks include creatures of most diverse forms 

 -^^ and habits, there are certain things Avhlch we may say of the 

 race as a wliole. 



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 l)ody 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 Avhich usually covers the back of the moUusk. 

 The feathery gills arise from the sides, and lie in the s]5ace 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 Chitonidce 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 maybe crawling, but if torn off they roll up into a ball. 

 A common species with us, about half an inch long, is TracJiy- 

 dermon apicidata, wiiich 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. 



