422 



PHYLUM MOLLUSC A. 



Of this myriad, about 9000 are streptoneural, the relatively 

 small minority are euthyneural Opisthobranchs and Nudi- 

 branchs, with light shells or none. The Heteropods and 

 some Opisthobranchs live in the open sea ; the great 

 majority of aquatic Gasteropods frequent the shore and 

 the sea bottom at relatively slight depths; the deep-sea 

 forms are comparatively few. 



Gasteropods rarely feed at such a low level as bivalves do 

 indeed, some of them are fond of 

 eating bivalves. Most Prosobranchs 

 (streptoneural), with a respiratory 

 siphon and a shell notch in which 

 this lies, are carnivorous, e.g. the 

 buckies (Bucdnuni) and * 'dog- whelks" 

 (Purpura) \ on the other hand, those 

 without this siphon, and with an 

 unnotched shell mouth, feed on 

 plants, e.g. the seaweed-eating peri- 

 winkles (Littorina). Most land 

 snails and slugs are vegetarian. 

 Many Gasteropods, both marine and 

 terrestrial, are voracious and indis- 

 criminate in their meals ; others are 

 as markedly specialists or epicures. 

 Some marine forms partial to Echino- 

 ^ ^ derms have a salivary secretion of 



FIG. 230. -Stages in mol- d u ilute sdphuric acid, which changes 

 luscan development. the carbonate of lime m the starfish 

 A., Biastuia of limpet (after i nto tn e more brittle and readily 



Patten). B, Gastrula of pulverised Sulphate. About ten 

 Paludina wvipara (after L r . . . . . . 



Tonniges); v., beginning of genera are parasitic on or in Echino- 

 ^t m mesoSr m a tefirT; derms > <* Sty lifer, Turtonia, Thyca, 

 later stage of the same ; ., and the extremely degenerate Ento- 

 concha, within the Holothurian Syn- 



aptd. Some Species of Eulima also 

 .f . ...... 



live a semi-parasitic life on certain 



Echinoderms. 



Life history. The eggs of Gasteropods are usually small, 

 without much yolk, but surrounded by a jelly, the surface 

 of which often hardens. In the snail and some others there 

 is an egg-shell of lime. 



teron; a., anus; 

 nmg of foot ; s 



gland. 



/., begin- 

 .g. , shell 



