AMPHINEURA. 



37 1 



paired cords the pedals are connected by numerous cross- 

 commissures, and the viscerals or pallials are united posteriorly 

 by a commissure above the rectum. The bilateral symmetry is 

 shown internally, e.g. in the paired nephridia, auricles, and 

 genital ducts. The class is of ancient origin, dating from the 

 Silurian. There are two orders — Polyp lacophora, e.g. 

 Chiton, and Aplacophora, e.g. Neomenia. 



1st Order of Amphineura. Polyplacophora (Chitonidse). 



The members of this order, represented on British coasts by several 

 species of Chiton, are sluggish, usually vegetarian, animals, occurring 



Fig. 165 — Anatomy of Chiton. 



A, ventral surface (after Cuvier). B, dorsal view ot alimentary 

 canal (after Lankester). C, genital and excretory organs from 

 dorsal surface (after Lang and Haller, diagrammatic). 711., 

 mouth ; a., anus ; br., numerous simple gills \f., foot ; b., buccal 

 mass ; /., liver ; z., intestine ; ao. t aorta ; z\, ventricle of heart ; 

 r.a. and La., right and left auricles ; ov., ovary ; od., oviduct ; 

 od'., opening of oviduct ; «., part of nephridium, represented in 

 black throughout ; no. , external opening of nephridium ; p. , 

 outline of pericardium. 



from the shore to great depths. The foot is generally as long as the 

 body ; the mantle covers the back and bears eight shell-plates (Fig. 162), 

 perforated, in many cases at least, by numerous sensory organs, which 

 may be in part optic ; numerous gills lie in a regular row along a groove 

 on each side between the mantle and the foot. 



In most cases the eight shell-plates are jointed on one another, and 

 the animal can roll itself up. The uncovered parts of the mantle bear 

 spicules. Ganglia, in the strict sense, are scarcely developed, but there 

 is a supra-cesophageal ganglionic commissure from which the visceral 

 and pedal cords extend backwards along the whole length of the body. 

 There are no special sense organs on the head, which is but slightly 

 differentiated ; but the pallial sense organs are usually numerous and 



