476 



PHYLUM MOLLUSCA 



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 com- 

 paratively few. 



Gasteropods rarely feed at such a 

 low level as bivalves do— indeed, 

 some of them are fond of eating 

 bivalves. Most Prosobranchs (strep- 

 toneural), with a respiratory siphon 

 and a shell notch in which this lies, 

 are carnivorous, e.g. the buckles 

 (Biiccinum) and " dog-whelks " (Pur- 

 pura) ; 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 terres- 

 FiG. 272.— Stages in ^^i^^' ^^^ voracious and indiscriminate 

 molluscan development, in their meals ; others are as markedly 

 A, Biastuia of limpet (after Specialists or cpicurcs. Somc marine 

 Patten). B, Gastruia of forms partial to Echinodcrms have a 



Paludina vivipara (after ,. ^ • r j-i .^ 1 u : , 



Tonniges) ; v., beginning of salivary sccretiou ot dilutc suiphuric 



velum; arc, archenteroii ; ^^J^ which chaUgeS the CarbouatC of 



m., mesoderm cells. C, ' . ^^ . . , 



Later stage of the same; i;., limC lU the Starhsh intO the morC 



guSto;,.,'"'»,r'"arr,'; brittle and readily pulverised sul- 



teron; a., anus ; /..begin- phate. About tcu geucra are para- 



ning of foot ; sh.g., shell ^. . • t^ i • j 



gland. sitic ou or m Echmoderms, e.g. 



Sty lifer, Tiirtonia, Thyca, and the 

 extremely degenerate Entoconcha, within the Holothurian 

 Synapta. Some species of Eulima also 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. 



