SYSTEMATIC 



CON"CHOLOaY. 



Subclass PULMONATA. 



This division embraces all the land and fresh-water mollusca 

 which breathe air. They are normal gastropods, having a broad 

 foot, and usually a large spiral, holostomate, inoperculate shell 

 (operculate in Amphibola). Their breathing organ is the sim- 

 plest form of lung, resembling the branchial chamber of the sea- 

 snails, but lined with a network of respiratory vessels. The 

 respiratory orifice is small and contractile to prevent too rapid 

 desiccation in the land-snails, and to exclude water in the aquatic 

 genera. Most of them have sufficiently large shells to contain 

 the animal ; in a few the shell only shelters a portion of the 

 animal, or it is internal and of simple structure, or rarely absent. 

 Snail-shells contain a larger proportion of animal matter than 

 sea-shells, and their structure is less distinctly stratified. The 

 Pulmonata are mostl}^ terrestrial, but some genera are fluviatile 

 and a few inhabit damp places near the sea, where at high-tide 

 they are covered by its waters. The sexes are united in each 

 individual, but the genital orifices are sometimes contiguous, 

 opening in a common cloaca, and sometimes distant. Through 

 the Cyclostomas or operculated land-snails and the Ampullariffi 

 they are related to the phytophagous sea-snails, through Siphon- 

 aria and Gadinia to the limpets, and through Onchidium to the 

 nudibranchs. 



Land-snails are universally distributed ; but the necessitj^ for 

 moist air, and the vegetable nature of their food, favor their 

 multiplication in warm and humid regions : they are espeoiallj' 

 abundant in islands, whilst in hot and desert countries they 

 appear only in the season of rain or dews. Their geological 

 history is less complete than that of the purely marine orders ; 

 but their antiquity might be inferred from the distribution of 

 peculiar genei-a in remote islands, associated with the living rep- 

 resentatives of the ancient fauna of Europe. Fresh-water snails 

 3 



