712 ZOOLOGY 



SECT. 



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

 morphological evidence but by their comparatively late appearance 

 in time. 



SINUPALLIATE 

 EULAMELLIBRANCHIA 



INTEGRIPALLIATE 

 EULAMELLIBRANCHIA 



PSEUDO-LAMELLIBRANCHIA 



HETEROMYARIAN 

 FILIBRANCHIA 



ISOMYARIAN 

 FILIBRANCHIA 



PROTOBRANCHIA 



Pig. 596. — Diagram illustrating the mutual relationships of the Pelceypoda. 



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 : 



