134 



GEOLOGICAL BIOLOGY. 



fossil species are described. The Opisthobranchia (Fig. 39) 

 are all sea-snails, and appear to be restricted in distribution to 

 the coastal waters, near the land, and near the line of contact 

 between salt and brackish water habitats; about 1200 species 



39 



FIGS. 36-42. Gastropoda Illustrations of the chief types ; 36, Placophora, Chiton ruber ; 37, 

 Scaphopoda, Dentalium Indianorum \ 38, 38^, Pulmonata, Physa heterostropha ; 39, 

 Opisthobranchia (Nudibranchia) sEolis pilata ; 40, 41, 42, Prosobranchia, 40, Cyclobran- 

 china, Achmaea testudinalis ; 41, Aspidobranchina, Haliotis sp. 42, Ctenobranchina, Turn'' 

 tella sp. (After Packard and McMurrich.) 



are described, including fossil forms; the gills are behind the 

 heart. The remaining order, the Prosobranchia (Figs. 40, 41, 

 42), includes mainly marine species, which are adapted to a 

 great variety of marine conditions; there are known some 

 14,000 species. They are divided into three suborders, sepa- 

 rated primarily upon the differences in their breathing organs, 

 viz.: A, Cyclobranchina; B, Aspidobranchina; C, Cteno- 

 branchina, or better known as the Pectinibranchia of Cuvier. 

 In all the Prosobranchs the gills are in front of the heart, 

 that is, the branchial vein enters from the front. They are 

 dioecious (while the Opisthobranchs and Pulmonates are 

 hermaphrodite). 



