GASTEROPODA. 



399 



FIG. i^. Chiton.- 

 After Pre"tre. 



the pedals are connected by numerous cross-commissures, and 



the viscerals or pallials are united posteriorly bv a commissure 



above the rectum. The bilateral svm- 



metry is shown internally, e.g. /;/ 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 Isopleura or Polyplacophora are 

 represented on British coasts by several 

 species of Chiton, sluggish, usually vege- 

 tarian, animals, 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. 198), 

 perforated, in many cases at least, by 

 numerous sensory organs, which ma} 1 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 - nesophageal 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, surrounded by a 

 diffuse digestive gland. There is a radula 

 in the mouth. The heart is median and 

 posterior, and consists of a ventricle and 

 two to eight auricles. There are two sym- 

 metrical nephridia opening posteriorly, and 

 consisting of much-branched tubes. The 

 sexes are separate ; a single reproductive 

 organ extends dorsally between gut and 

 intestine almost the whole length of the 

 body ; the genital ducts are paired and 

 open posteriorly in front of the excretory 

 apertures. The ova, with chitinous spiny 

 shells, are usually retained for some time by 

 the female between the mantle and the 



FIG. 199. Dorsal view 

 of nervous system of 

 Chiton. - - After Pel- 

 seneer. 



c. , Cerebral commissure ; 

 -., gut (above all the com- 

 missures except cerebral 

 and supra- rectal) ; pa. , 

 pallial or visceral loop, 

 with supra - rectal com- 

 missure (s.r.c.) ; /., pedal 

 nerves united by numer- 

 ous transverse branches ; 

 s.g:, stomato-gastric com- 

 missure ; s.r., subradular 

 commissure ; /., labial 

 commissure ; r., visceral 

 commissure. 



