CHAPTER IX. 



CLASSES, ORDERS, AND SUB-ORDERS, 



THE mollusca are divided into four classes : 



1. Cephalopoda (head-footed), having the foot in the form of 



arms around the head. 



2. Gastropoda (belly-footed), having the foot beneath the body. 



3. Scaphopoda (spade-footed), "having the foot more or less in 



the shape of a spade. 



4. Pelecypoda (axe-footed), having the foot more or less in the 



shape of an axe. 



The Cephalopoda, as stated in Chapter III., are divided into 

 two orders : 



1. Dibranchiata, those with two gills. 



2. Tetrabranchiata, those with four gills. 



The Gastropoda are divided into four orders, the first con- 

 taining such forms as have a multivalve shell and are bilaterally 

 symmetrical, the other three being comprised of those which have 

 the shell univalve or absent, and are asymmetrical. Thus we have: 



Shell multivalve 



1. Amphineura. 



Shell univalve or absent 



2. Prosobranchiata gills in front of heart, visceral loop 



twisted, one pair of tentacles, shell generally operculated ; 

 dioecious. 



3. Opisthobranchiata gills behind heart, visceral loop not 



twisted and shell not operculated (except in Act&ori), foot 

 prolonged into lateral epipodia or into parapodia ; 

 monoecious, 



4. Pulmonata air breathing, visceral loop not twisted and 



shell not operculated, two pairs of tentacles ; monoecious. 

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