CLASSIFICATION OF GASTEROPODA 



Thus we have 



MOLLUSCA 



Glossophora 



Cephalopoda Gasteropoda Scaphopoda 



Aglossa 



Pelecypoda 



Classification of Gasteropoda. — The Gasterojwda are nu- 

 merically very largely in excess of the two other Orders of the 

 Glossophora, far more complicated as regards classification, and 

 contain a large proportion of those examples of the Mollusca 

 which are most familiar to the ordinary observer. It will there- 

 fore be convenient to postpone for the present a fuller discussion 

 of the subdivisions of the Cephalopoda and Scaphopoda, as well 

 as of the Aglossa, returning to them again in special chapters 

 (chaps, xiii. and xvi.), and to devote a few introductory words to 

 the classification and relations of the Gasteropoda. 



The Gasteropoda are divided into four Classes, Am^Jhineura, 

 Prosohranehiata, Opistliohranchiata, and Pulmonata. 



(1) The Amphineura'^ are bilaterally symmetrical Mollusca, 



Fig. 3. — An example of the 

 Aplacophora, Ncomenia ca- 

 rinata Tullb. : a, anus ; g7\ 

 ventral groove ; m, mouth. 



Fig. 2. — An example of the 



Polyplacophora : Chiton spin- 

 osus Brucr. 



i.e. with organs either single and central, or paired and disposed 



on either side of the longer axis of the animal. The shell, when 



1 a.ix<pl, on both sides ; vevpov, nerve, vessel. Some authorities regard the 

 Amphineura as a distinct Order. 



