BRITISH MOLLUSCA. 



GASTEROPODA PULMONIFERA. 



All the terrestrial, and the majority of freshwater 

 Gasteropodous Mollusca, breathe air, and are provided 

 with a pulmonary cavity, or sac, instead of gills. Over 

 the walls of this sac the blood is distributed in minute 

 vessels, for the purpose of being aerated. The animals 

 presenting this organization, are all provided with dis- 

 tinct heads, furnished with tentacula and organs of sight. 

 They walk by means of a well-developed creeping disk. 

 The majority are protected by shells, but there are 

 some which have no shell, although they do not differ 

 materially in organization or external form. The shells 

 when present are, in the majority of cases, spiral or 

 discoid, and external, in a few instances patelliform ; 

 when imbedded in the substance of the mantle, they 

 are unguiform. The embryos have, or have not, shells, 

 according as the adult is to be shell-bearing or naked. 

 Distinct ciliated lobes are not seen upon them, though 

 in the egg they are partially clothed with cilia on the 

 analogous portion of their bodies, and exhibit movements. 

 The number of known Pulmoniferous Mollusks is very 

 great. They inhabit all climates, and display variations 

 of colour and form that bear a remarkable relation to 



VOL. iv. b 



