SURVEY OF INVERTEBRATES 



87 



for locomotion ; and a mantle cavity between the main body and 

 an enclosing envelope, the mantle. Among the five classes of 

 Molluscs only three are of sufficient general interest to command 

 our attention here: the Gastropoda, Pelecypoda, and Cephalopoda. 

 Gastropoda. One usually thinks of Molluscs as sea-dwelling 

 animals but among the some sixteen thousand species of Snails, 

 Slugs, and other similar Gastropods, more than one-third are 

 terrestrial. Everyone is familiar with the spirally-coiled Snail's 

 shell into which the animal can completely retire when disturbed, 

 but many close relatives, such as the Limpets, have shells that are 

 merely simple flattened cones, while most of the Slugs have no 

 shell at all. The shell when present is secreted chiefly by the in- 



Intestine, 



Shell 



Mantle cavity 

 Mantle 



Body 

 in shell 



Tentacles 



t .Mouth 



Intestine 

 Respiratory aperture B 



Fig. 48. 



A Anus'' 



A, Chiton, ventral view; B, Snail, diagrammatic side view 



conspicuous mantle. The common Snails and Slugs glide along on a 

 path of slime by muscular contractions of the foot which forms the 

 entire ventral part of the body, but some of the marine species ac- 

 tively swim by means of delicate, undulating expansions of the foot. 



The Snails and their allies have a well-developed head with 

 tentacles and eyes, and a mouth, supplied with a unique rasping 

 tongue-like organ, the radula, that leads into a complicated di- 

 gestive tract. Add to the digestive organs the complex blood vascu- 

 lar system, excretory system, nervous system, and reproductive 

 system, and it becomes evident that even the lowly Garden Slug 

 belies its soft slimy body. (Fig. 48.) 



Pelecypoda. However, the Mollusca is a phylum of surprises, 

 for the Pelecypoda is a class of headless animals that for the most 

 part have taken to a sedentary life within a shell composed of two 

 valves hinged together: they are Bivalves. The adult Oysters 

 are permanently attached and therefore footless; the Clams are 



