472 



PHYLUM MOLLUSCA 



ganglionic cells but at most very slightly developed ganglia, 

 which run the whole length of the body. Of these 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 

 is one order — Polyplacophora, e.g. Chiton. 



The Isopleiira or Polyplacophora are represented on British coasts 

 by several species of Chiton, sluggish, usually vegetarian, animals, 



Fig. 270. — Anatomy of Chiton. 



Ventral surface (after Cuvier). B, Dorsal view of alimentary 

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

 dorsal surface (after Lang and Haller, diagrammatic), m., Mouth ; 

 a., anus; br., numerous simple gills; /., foot ; b., buccal mass; 

 /., liver; i., intestine; ao., aorta; v., ventricle of heart; r.a. 

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

 opening of oviduct ; n., part of nephridium, represented in black 

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

 pericardium. 



occurring 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. 268), perforated, in many cases at least, by numerous 

 sensory organs, which are 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-oesophageal 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 varied. A twisted gut runs through the body, sur- 



