SURVEY OF GASTEROPODS. 315 



nervous system and other parts. In a large number, the 

 visceral hump on the back is twisted spirally, and the adult 

 shell likewise. Not a few, however, especially free-swimmers, 

 are superficially symmetrical, and the adult shell may be 

 cap-like, degenerate, or absent. The head-region is well- 

 developed and symmetrical ; it contains several concen- 

 trated pairs of ganglia and bears sense-organs. The mouth 

 contains a rasping ribbon or radula. The foot is sym- 

 metrical. It is usually a flat median creeping sole, and 

 generally contains a large mucous gland. 



Among these asymmetrical Gasteropods, there is great 

 variety of form and habit. 



{a) The Streptoneura, in which the visceral nerve-loop is 

 implicated in the visceral torsion and twisted like the figure 

 8, in which, moreover, the sexes are separate, include 

 limpets {Patella), ear-shells (Haliotis), the vegetarian pond- 

 snail (Paludina), the carnivorous " buckie " {Buccinum), the 

 sea- weed eating periwinkle (Littorina\ the cowries {Cyprced), 

 and many others, besides the divergent free-swimming 

 carnivorous Heteropods, e.g., Atlanta, Carinaria, Ptero- 

 trachea. 



(b) In the Euthyneura, the visceral nerve-loop is not 

 twisted but often very short ; the shell is generally light and 

 sometimes absent in the adult ; the food-canal is not twisted 

 so far forwards as in (a), and sometimes ends medianly and 

 posteriorly ; the reproductive organs, moreover, are herma- 

 phrodite. The sea-hare {Aplysia), such nudibranchs as 

 Doris and Eolis, are types of what are often called opistho- 

 branchs (gill behind the heart). But the Euthyneura also 

 include many forms without gills in which the mantle cavity 

 is a respiratory chamber, as in the fresh- water snails (e.g., 

 Planorbis, Limnceus), the land snails {Helix), the slugs 

 {Limax, Arion). 



Apart from these Gasteropods, and yet not so far apart as 

 to form a separate class, many authorities rank the sea- 

 butterflies or Pteropods, which we shall notice after- 

 wards. 



Mode of Life. — From the number of diverse types which 

 we have already mentioned, it is evident that few general 

 statements can be made about the life of Gasteropods. We 

 are safe in saying, however, that though the majority are 



