MOLLUSCA 277 



595. Order i. — The Gastropoda have a distinct head, a twisted visceral 

 sack, a single shell and a creeping foot with, sometimes, lateral swimming 

 lobes. The head usually bears two or four tentacles and a pair of eyes. 

 The visceral organs are chiefly contained in a dorsal conical sack-like 

 development of the body-wall which is more or less coiled in form of a 

 spiral. The lower edge of the mantle forms a collar-like continuation of 

 the ventral edge of the visceral sack. The entire surface of the visceral 

 sack down to the edge of the collar is covered with a calcareous shell into 

 which the head and foot may also be withdrawn. Between the surface 

 of the mantle and the body there is a space of considerable size. This is 

 called the mantle cavity. In it lie two feather-like gills, ctenidia, which 

 are developed from the body-wall. Because of the twisting of the vis- 

 ceral mass the gills become shifted in position and one is frequently 

 reduced. 



s\. 



a. 



7 — i 



Fig. 164. — The garden snail, Helix. A, The shell in section, a, Apex; </;/. 

 anus; ap. aperture; c, columella; e, eyestalk; /, foot; /, lip; m, edge of mantle 

 (collar); ra, respiratory aperture; s, suture; /, tentacles. (From Galloway.) 



596. In the mouth cavity there is a horny jaw and a tongue covered 

 with a rough, file-like cuticular ribbon. This as called a radula. The 

 digestive tract is rather long and coiled. There are a pair of "salivary" 

 glands with ducts opening into the buccal cavity. The oesophagus is 

 enlarged into a crop. Another enlargement of the canal forms a stomach 

 into which the ducts of a large digestive gland ("liver") open. The 

 coiled small intestine opens into a shorter and wider large intestine. 



