LIFE-HISTORY OF GASPEROPODS 477 



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 " 

 {Biiccinum iindatum) united in a ball about the size of an 

 orange. Under the ledges of rock are many little vases or 

 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 development is usually simple and typical. In other 

 words, segmentation is total though often unequal ; gastru- 

 lation is embolic or epibolic according to the amount of yolk 

 present ; the gastrula becomes a trochosphere, and later a 

 veliger (Fig. 272, C). 



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

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

 rocks ; thence onwards they increase. Most of the Palaeozoic 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 Palaeozoic ages, and the terrestrial air-breathers 

 are comparatively modern. 



Ecology. — As voracious animals, with irresistible 

 raspers, Gasteropods commit many atrocities in the 

 struggle for existence, and decimate many plants. Pro- 

 fessor Stahl shows, however, that there are more than a 

 dozen different ways in which plants are saved from snails 

 —by crystals, acids, ferments, etc. ; in short, by consti- 

 tutional characteristics sufficiently important to determine 

 survival in the course of natural selection or ehmination. 

 As food and bait, many Gasteropods are very useful ; their 

 shells have supplied tools and utensils and objects of 

 delight ; the juices of Purpura and Murex furnished the 

 Tyrian purple, more charming than all aniline. 



