260 



COLLEGE ZOOLOGY 



prevent the evaporation of moisture from their bodies. Plan- 

 orbis (Fig. 185, B) differs from Physa and Lymncea in having a 



shell coiled in one plane 

 like a watchspring. 



Marine Gastropods. 

 The majority of the 

 marine gastropods have 

 shells, but many of them 

 do not ; some of the 

 latter are called nudi- 

 branchs. Littorina lit- 

 torea, the periwinkle, is 

 a very common shelled 

 snail on the North At- 

 lantic sea-shore. It was 



FIG. 186. - Two marine GASTROPODS. i ntro duced from Europe, 

 A, Sycotypus camculatus. B, Crepidula. 

 (A, from Davenport; B, from Weysse.) where in many localities 



it is used as an article of 



food by the natives. In Crepidula (Fig. 186, B) the spiral has 

 'almost disappeared, and the shell is boatlike. Acm&a, the 

 limpet, is a sea-snail modified so as to cling closely to rocks. Its 

 shell is conical. In Europe limpets are used as food. Sycotypus 

 (Fig. 1 86, A) is a very large marine gastro- 

 pod that lives in shallow water and feeds 

 on other mollusks. Urosalpinx, the oyster 

 drill, and several other marine snails, make 

 a practice of boring through the thick shells 

 of oysters and other bivalves with their 

 radulas and taking out the soft body of 

 the victims through the hole. 



The term nudibranch is applied to certain 

 shell-less marine gastropods. The nudi- 

 branchs resemble the terrestrial slugs ; 

 they do not breathe air, however, but take 

 oxygen from the water by means of naked Davenport.) 



