346 



MOLLUSC A, 



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 



ST.C 



Fig, 113. — Dorsal view 

 of nervous system of 

 Chiton. (After Pel- 



SENEER. ) 



c. , Cerebral commissure; 

 g., gut (above all the com- 

 missures except cerebral and 

 supra-rectal); pa.^ pallial or 

 visceral loop with supra- 

 rectal commissure {s.r. c.')\ 

 /. , pedal nerves united 

 by numerous transverse 

 branches ; s.g;.^ stomato- 

 gastric commissure ; s,r., 

 subradiilar commissure; /., 

 labial commissure; v., vis- 

 ceral commissure. 



Fig. 114. — Proneomenia. Ner- 

 vous system { From HuB- 

 RECHT. ) 



c.g.^ Cerebral ganglia; slg.., sub- 

 lingual ; a.p.g.y anterior pedal ; p-p-g-, 

 posterior pedal; p.7\g., posterior vis- 

 cerals ; si., sublingual connectives; 

 cfic.^ cerebro-pedal connective; pe., 

 longitudinal pedal ner\'es ; /a., longi- 

 tudinal lateral nerves. 



differentiated ; but ihe i^diial sense organs are usually numerous and 

 varied. A twisted gut I'uns through the bod)-, 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 2-S auricles. Numerous 

 gills lie in a regular row along a groove on each side between the mantle 

 and the foot. There are two symuietrical nephridia opening posteriorly. 



