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 {Buccininn) 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., Euliina 

 and Stylifer on Echinoderms, and 

 the extremely degenerate Efitoconcha 

 inirabilis, — 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. 



.4 , Blastula of Limpet (after 

 Patten); B, Gastrula of /'o/w- 

 (//war'/V'z^^izra (after Tonniges): 

 z\ beginning of velum ; arc, 

 archenteron ; ?«, mesoderm 

 cells ; C, later stage of the 

 same ; z\ velum ; tn, mouth 

 invagination ; arc. archen- 

 teron : a, anus ; /, beginning 

 of foot ; sh.g, shell gland. 



