3i8 MOLLUSCS. 



older the Gasteropods have increased in numbers. A few 

 have been disinterred from Cambrian rocks ; thence onwards 

 they increase. Most of the Palaeozoic 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 Palaeozoic ages, and the 

 terrestrial air-breathers are comparativelymodern. 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. 



General Interest. — I do not like to leave the discussion of 

 Gasteropods without at least saying that their shells are often 

 very beautiful in colour and graceful in form, the reason of 

 which can be found only in that harmony which fills the 

 heart of things. 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. ; and like an orthodox Darwinian he 

 regards these plants as the survivors of a multitude, which 

 did not become sufficiently gritty or poisonous. 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. 



A type of G.4STER0P0DA — The garden snail {Helix aspersa), 

 or the edible snail {H. pomaiia). 



Mode of Life. — Our very common garden-snail {II. aspersa) 

 and its larger neighbour species {H. pomatia), rare in Eng- 

 land but abundant on the continent, are so like one another 

 except in size, that the same description will serve for both. 

 They are thoroughly terrestrial animals, breathing air directly 

 within a pulmonary chamber, and are slowly drowned when 



