376 



MOLLUSC A. 



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

 periwinkles (Littorina). 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 



Fig. 166. — Stages in mol- 

 luscan development. 



a, Biastuia of limpet (after inside the genital duct. Development 

 Patten), b, Gastmia of sometimes proceeds within the parent, 



Faludina vivipara (after . '- ' 



stage of the same ; 

 velum; m., mouth inva- 

 gination ; arc, archen- 

 teron ; a., anus;^, begin- 

 ning of foot ; sh.g., shell 

 gland. 



T6nniges);w., beginning of but in most cases the fertilised eggs 

 Sm'esoS «£™r are laid in gelatinous clumps, or within 

 special capsules. The free-swimming 

 Ianthina 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 " (Buctinum undatum) 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, 

 and in some other forms, there is a struggle for existence 

 — an infant cannibalism — in the cradle, for out of the 

 numerous embryos in each capsule only a few reach 



