712 ZOOLOGY SECT 



upon as the highest members of the class is indicated, not only 

 morphological evidence but by their comparatively late appearanc 

 in time. 



SINUPALLIATE 

 EULAMELLIBRANCHIA 



INTEGRIPALLIATE 

 EULAMELLIBRANCHIA 



PSEUDO-LAMELLIBRANCHIA 



HETEROMYARIAN 

 FILIBRANCHIA 



ISOMYARIAN 

 FILIBRANCHIA 



PROTOBRANCHIA 



FIG. 596. Diagram illustrating the mutual relationships of the Pelecypoda. 



CLASS II -AMPHINEURA. 



The Amphineura are a class of marine Mollusca formerly 

 grouped with the Gastropoda, but now recognised as sufficiently 

 far removed from the latter to require separation as a distinct 

 class. The commonest, as well as the most highly organised, of 

 the Amphineura are the Chitons, a group of remarkably sluggish 

 Limpet-like Molluscs with a shell composed of eight pieces. The 

 other members of the class are lowly organised, comprising the 

 most primitive forms of the entire phylum, all of which are devoid 

 of a shell. 



1. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Amphineura may be defined as bilaterally symmetrical, 

 more or less elongated Mollusca, with terminal mouth and anus, 

 either devoid of a shell, or possessing one which consists of eight 

 median valves. The mantle contains numerous spicules of carbon- 

 ate of lime, and is not divided into paired lateral lobes. The 

 ctenidia are either absent, or there is a single pair, or they 

 occur as a circlet round the anus, or as two lateral rows situated 

 between the edge of the mantle and the side of the foot. A 

 radula (vide infra) is sometimes present, sometimes absent. 

 The nervous system consists of two pairs of nerve-cords, pedal and 

 pallial, connected in front with a nerve-ring. 



The class is divisible into two orders : 



