418 PHYLUM MOLLUSCA. 
eg. 
Fic. 226.—Chiton,— 
After Prétre, 
Fic. 227.—Dorsal view 
of nervous system of 
Acanthochiton.—After 
Pelseneer. 
x, Upper buccal commissure ; 
2, upper buccal ganglion ; 
3, stomatogastric commis- 
sure; 4, labial commissure; 
5, sub-radular ganglia; 6, 
anterior pedal commissure } 
7, pedal nerve with pallio- 
pedal connections; 8, 
supra-rectal pallial com- 
missure ; 9, pallial nerve; 
10, anastomosis of branches 
ofpedalnerves; r1,stomato- 
gastric ganglia; 12, ceso- 
phageal nerves; 13,cerebral ; 
cominissure, 
in the paired nephridia, auricles, and genital ducts. 
The class, is of ancient origin, 
dating from the Silurian, There 
is one order—Polyplacophora, ©. 
Chiton. - 
The Isopleura or Polyplacophora are 
represented on British coasts by several 
species of Chzton, 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. 
226), 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-cesophageal gangli- 
onic 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 numer- 
ous 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 pos- 
terior, and consists of a ventricle and 
two to eight auricles. There are two 
symmetrical nephridia opening posteriorly, 
and consisting of much-branched tubes, 
The sexes are separate; a single repro- 
ductive organ extends dorsally between 
gut and aorta 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 re- 
tained for some time by the female be- 
tween the mantle and the gills. The 
.segmentation is holoblastic, and a gastrula 
is formed by invagination. 
