4 i8 



PHYLUM MOLLUSC A. 



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 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. 

 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. 



FIG. 227. Dorsal view 

 of nervous system of 

 Acanthochiton. After 

 Pelseneer. 



i , 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 

 ofpedal nerves ; u,stomato- 

 gastric ganglia ; 12, ceso- 

 phageal nerves; i3,cerebral 

 commissure. 



