36o 



MOLLUSC A. 



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 (Bucdnum) and " dog whelks " (Purpura) ; on the 

 other hand, those without this siphon, and with an un- 

 notched shell mouth, feed on plants, e.g., the seaweed eating 

 periwinkles (Littorina). The vege- 

 tarian habits of most land snails 

 and slugs are known to all. Many 

 Gasteropods, both marine and ter- 

 restrial, are very voracious and in- 

 discriminate in their meals; others 

 are as markedly specialists or epi- 

 cures. Some marine forms, partial 

 to Echinoderms, have got over the 

 difficulty of eating such hard food 

 by secreting dilute sulphuric acid, 

 which is said to change the car- 

 bonate of lime in the starfish into 

 the more brittle and readily pul- 

 verised sulphate. Only a few Gas- 

 teropods are parasitic, e.g., Eulima 

 and Sty lifer on Echinoderms, and 

 the extremely degenerate Entoconcha 

 mirabilis, within the Holothurian 

 Synapta. 



Life History. 



The eggs of Gasteropods are 

 usually small, without much yolk, 

 but surrounded by a jelly, the sur- 

 face of which hardens. In the 

 snail and in some others there is 

 an egg shell of lime. 



Sexual union occurs between her- 

 maphrodites as well as between separate sexes, and ferti- 

 lisation is effected inside the genital duct. Development 

 sometimes proceeds within the parent, but in most cases 



FIG. 117. Stages in 

 Molluscan development. 



A , Blastula of Limpet (after 

 Patten) ; B, Gastrula of Palu- 

 dina7rivtyara.(a.fterTdnniges)', 

 v, beginning of velum ; arc.) 

 archenteron ; ;>z, mesoderm 

 cells ; C, later stage of the 

 same ; z/, velum ; in, mouth 

 invagination ; arc., archen- 

 teron ; a, anus ; f, beginning 

 of foot ; sh.g, shell gland. 



