694 ZOOLOGY SECT. 



branchia, lastly, there is a gradual series of stages from compara- 

 tively generalised forms with free mantle-lobes up to the highly 

 specialised species with large siphons. That the Pseudolamelli- 

 branchia and the siphoniate Eulamellibranchia are to be looked 

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

 morphological evidence, but by their comparatively late appearance 

 in time. 



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 simplest forms, all devoid 

 of a shell, of the entire phylum. 



1. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Amphineura may be denned 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 : 



ORDER 1. PLACOPHORA. 



Amphineura with a broad foot, and with a shell which consists 

 of eight transverse valves. There is a row of ctenidia on either side. 

 This order includes the Chitons. 



ORDER 2. APLACOPHORA [SOLENOGASTRES]. 



Amphineura with an elongated body covered completely by the 

 mantle, without shell, but with calcareous spicules. There is no 

 foot, but generally a ventral longitudinal groove along which 

 usually runs a low ciliated ridge. In some there is a posterior 

 cavity (cloaca or mantle-cavity) containing a pair or a circlet 

 of ctenidia. 



This order includes Neomenia, Proneomenia, Chcetoderma, and 

 a number of other genera. 



