292 



COLLEGE ZOOLOGY 



Seminal receptacle 



Hermaphroditic duct 

 Ovotestis 



Albumen gland 



Digestive 

 gland 



Eye 



Tentacle 



Genital pore 

 Vagi 



Mouth 

 Cerebral ganglia 



Penis 



Salivary gland 



Figure 181, Internal structure of a snail; dorsal view. 



Fresh-water snails are numerous in creeks 

 and pools. Snail shells may coil in two direc- 

 tions—clockwise or counterclockwise. These 

 are distinguished by the terms dextral and 

 sinistral. The type of coiling can be deter- 

 mined by holding the shell with the aperture 

 toward the observer: if the opening is on 

 the right as in Busycon canaliculatum (Fig. 

 182), then it is dextral, but if on the left, 

 then it is sinistral. The difference is the 

 same as that between a "right-handed" and 

 a "left-handed" screw. Some fresh-water 

 snails possess gills with which they breathe 

 under water, others are pulmonates and 

 have a lung cavity so that they must come 

 to the surface from time to time for air 

 when the water is warm, otherwise cutane- 

 ous respiration may be adequate. Many 

 gastropods— land, fresh-water, and marine — 

 serve as intermediate hosts for blood and 

 liver flukes. 



Land slugs (pulmonates) are closely re- 

 lated to the land snails, but are completely 

 naked. To keep from drying up, slugs must 

 live in a moist place such as is afforded 

 under boards and stones and in holes in the 

 ground. At night they feed on vegetation. 

 Some sea slugs (nudibranchs) live among 

 seaweeds, which they may resemble so 

 closely that it is practically impossible to see 

 them, while others are quite conspicuous. 



Whelks and periwinkles are among the 

 commonest of the smaller marine snails. 

 The largest marine snail in America is the 

 queen conch which lives in the Atlantic, 

 being especially common along the shores 

 of the Florida Keys and the West Indies. 

 The shell may be a foot long and weigh 5 

 pounds. Also called a conch is the channel 

 shell which hermit crabs find very satisfac- 

 tory as a place in which to live. The spiral 

 characteristic of the snail shell is absent 



