LIFE HISTORY. 



403 



a shell notch in which this lies, are carnivorous, e.g. the 

 buckies (Buccinum] 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 (Littorind]. The vegetarian habits of most land 

 snails and slugs are known to all. Many Gasteropods, both 

 marine and terrestrial, are very voracious and indiscriminate 

 in their meals; others are as markedly specialists or epicures. 

 Some marine forms partial to Echino- 

 derms, have got over the difficulty of 

 eating such hard food, by secreting 

 dilute sulphuric acid, which changes 

 the carbonate of lime in the starfish 

 into the more brittle and readily pul- 

 verised sulphate. A few Gasteropods 

 are parasitic, e.g. Eulima and Stylifer 

 on Echinoderms, and the extremely 

 degenerate Entoconcha mirabilis,- 

 within the Holothurian Synapta. 



Life history.- -The eggs of Gastero- 

 pods 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. 



Sexual union occurs between her- 

 maphrodites as well as between separ- 

 ate sexes, and fertilisation is effected FlG - 201. Stages in mol- 

 inside the genital duct. Development luscan devel P ment 

 sometimes proceeds within the parent, 

 but in most cases the fertilised eggs 

 are laid in gelatinous clumps, or within 

 special capsules. The free-swimming 

 lanthina carries the eggs in capsules 

 attached to a large raft - like float 

 towed by the foot. On the shore one 

 often finds numerous egg-capsules of 

 the " buckie " (Bucdmtm undalnm] united in a ball 

 about the size of an orange. Under the ledges of rock 

 are many little yellowish cups, the egg-capsules of the 

 dog-whelk (Purpura lapillus]. In the buckie and whelk, 



c 



Paludina vivipara. (after 

 Tonniges); v., beginning of 

 velum; arc., archenteron ; 

 in., mesoderm cells. C, later 

 stage of the same; v. , 

 velum; ;//., mouth inva- 

 gination ; an., archen- 

 teron ; a., anus ;f., begin- 

 ning of foot ; sh.g., shell 

 gland. 



