LIFE-HISTORY OF GASTEROPODS. 317 



meals ; others are as markedly specialists or epicures. 

 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 carbonate of lime 

 in the starfish into the more brittle and readily pulverised 

 sulphate. Only a few Gasteropods are parasitic, e.g., 

 Eulima and Stylifer on Echinoderms, and the extremely 

 degenerate Entoconcha mirabilis, — within the Holothurian 

 Synapfa. 



Life-History. — The eggs of Gasteropods are usually small, 

 without much yolk, but surrounded by a jelly, the surface of 

 which hardens. In the snail and in some others there is 

 an egg-shell of lime. 



Sexual union occurs between hermaphrodites as well as 

 between separate sexes, and fertilisation is effected inside the 

 genital duct. Development 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 " 

 [Bucdnum 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 cajwule 

 only a few reach maturity, those that get the start eating the 

 others as they develop. 



The segmentation of the ovum is total, but somewhat 

 unequal ; a gastrula is formed by invagination or by over- 

 growth according as there is less or more yolk ; the gastrula 

 becomes a trochosphere with a pre-oral ring of cilia ; the 

 trochosphere grows into a veliger with a lobed ciliated 

 cushion or velum, a visceral dome, a dorsal shell-gland 

 which soon disappears, and an incipient ventral foot. In 

 terrestrial snails like Helix, the life-history is abbreviated. 

 In the water-snail Limnceus, Ray Lankester has detected the 

 persistence of the velum in the circumoral lobes of the 

 adult. 



Past History of Gasteropods. — As the earth has grown 



