BIONOMICS. 361 



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 numer- 

 ous egg capsules of the "buckie" {Bumnnm nndatum) 

 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 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 Limnaus, Ray Lankester has detected 

 the persistence of the velum in the circumoral lobes of the 

 adult. 



Past History. — As the earth has grown older the Gasteropods have 

 increased in numbers. A few have been disinterred from the Cambrian 

 rocks ; thence onwards they increase. Most of the Paleozoic genera are 

 now quite extinct, but many modern famihes trace their genealogy to the 

 Cretaceous period. Those with respiratory siphons were hardly, if at 

 all, represented in Palasozoic ages, and the terrestrial air breathers are 

 comparatively modern. Zoological statisticians estimate the number of 

 Gasteropods at 23,000, of which 7000 are extinct, 16,000 alive. But 

 besides the numerical success which may be inferred from these figures, 

 it is important to notice that not a few types have persisted from early 

 ages. 



Bionomics. 



As voracious animals, with irresistible raspers, Gastero- 

 pods commit many atrocities in the struggle for existence 

 and decimate many plants. Professor Stahl shows, however, 

 that there are more than a dozen different ways in which 

 plants are saved from snails, — by crystals, acids, ferments, 

 &c. ; and like an orthodox Darwinian he regards these 



