LIFE HISTORY—@GCOLOGY. 423 
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 Janthina 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” 
(Buccinum undatum) 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 iynical In other 
words, segmentation is total though often unequal ; gastrula- 
tion is embolic or epibolic according to the amount; of yolk 
present ; the gastrula becomes a trochosphere, and later a 
veliger (Fig. 230). 
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 increasé. Most of the Paleozoic genera 
are now quite extinct, but many modern families trace their genealogy 
to the Cretaceous period. Those with respiratory siphons were hardly, 
if at all, represented in Paleozoic Agee, and the terrestrial'air-breathers — 
are comparatively modern. 3 
CEcology.— As voracious animals, with irresistible 
raspers, Gasteropods 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, etc.; in short, by constitutional 
characteristics sufficiently important to determine survival 
in the course of natural selection or elimination. 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. 
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