GEOGRAPHICAL DISTRIBUTION. 133 



7th. Fresh water, as in rivers and lakes. 



8th. Amphibious conditions, fresh water and land. 



9th. Land, the surface of the land, or in the air. 



These are zones of environment, which express a series of 

 varying conditions of light, of oxygen, of air, of moisture, of 

 degrees of temperature, of pressure of the medium, of depth, 

 of height. 



Reasons for Selecting the Gastropods. The Gastropoda are 

 selected because of the wide range of adaptation expressed in 

 their distribution, and because the statistics are particularly 

 full. The classification found in Zittel's Handbuch is adopted, 

 so far as nomenclature and inclusion of genera are concerned ; 

 but Gastropoda will be spoken of as of the rank of a class, the 

 more common usage of zoologists,* and the morphologically 

 specialized forms, the Chitons (Placophora, Fig. 36) and the 

 Dentalia (Scaphopoda, Fig. 37), will be omitted from the true 

 Gastropoda, as is done by Zittel, following Ihring and Lacaze 

 Duthiers : the Pteropoda will also be omitted. 



Peculiarity of the Divisions of the Gastropods as to Range of 

 Adaptation. Ranking Gastropoda as a class, with the restric- 

 tions above mentioned, it will include the following four 

 orders, viz. : Prosobranchia, Opisthobranchia, Pulmonata, 

 and Nucleobranchiata (or Heteropoda). The whole of the 

 Heteropoda are specialized in structure and restricted in dis- 

 tribution to the surface and upper parts of the ocean water, 

 and structurally they may be ranked with the monotocardian 

 Prosobranchs. Six living genera with about 50 species are 

 known, and a few fossil genera are referred to this order. 

 The Pulmonata (Fig. 38, 38^) are air-breathers, and (with the 

 exception of the Siphonaridae) are restricted in distribution to 

 land and fresh water. Six thousand (6000) living and 700 



* Lankester's classification is (Encycl. Brit., art. " Mollusca," p. 633): 

 Phylum mollusca : 



Branch A, Glossophora. 



Class i. Gastropoda. Class 3. Cephalopoda. 



Br. a. Isopleura. Br. a, Pteropoda. 



Br. b. Anisopleura. Br. b. Siphonopoda. 



Class 2. Scaphopoda. 



Branch B, Lipocephala ( = Acephala, Cuvier). 

 Class i. Lamellibranchia (syn. Conchifera). 



