8 



CLASSIFICATION OF GASTEROPODA 



Thus we have 



MOLLUSCA 



I 



Glossophora 



I 



I i I 



Cephalopoda Gasteropoda Scaphopoda 



I 



Aglossa 



Pelecypoda 



Classification of Gasteropoda. — The G-asteropoda 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 MoUusca 

 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, AmpJdneura, 

 Prosohranchiata^ Opisthohranchiata^ and Pulmonata. 



(1) The Amphineura^ are bilaterally symmetrical MoUusca, 



Fig. 3. — An example of the 

 Aplacophora, Neonienia ca- 

 Hnata Tullb.: a, anus; gr, 

 ventral groove; m, mouth. 



Fig. 2. — An example of the 

 Polyplacophora : Chiton spln- 

 osus Brug. 



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 dfx(pl, on both sides ; veOpov, nerve, vessel. Some authorities regard the 

 Amphineura as a distinct Order. 



