xii PHYLUM MOLLUSCA 663 



CLASS IL 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 forms, comprising 

 the most primitive 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. An 

 odontophore (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. 



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 containing a pair or a circlet of ctenidia. 



This order includes Neomenia, Proneomenia, Chcetoderma, and 

 several allied genera. 



2. GENERAL ORGANISATION. 



External Features. The Aplacophora are distinguished 

 by their worm-like body, sometimes elongated and narrow and 

 capable of being coiled into a spiral, sometimes comparatively 



