ISOPLEURA. 



389 



VIII 



--VI 



It has a pair of small ganglia posteriorly near the anterior part 

 of the stomach. 



The stomatogastric commissure, with its contained buccal ganglia, 

 arises from the point of division of the cerebral into pallial and 

 pedal trunks, and passes between the buccal mass and oesophagus. 

 Finally, there is another suboeso- 

 phageal commissure with a pair of 

 small ganglia arising from the 

 labial commissure and supplyir 

 the subradular organ. - ^^* t - L 



Sense organs. The edges of the 

 snout or head are extended into 

 short labial palps. On the floor 

 of the mouth just in front of the 

 radula is a small epithelial projec- 

 tion, which is supposed to be a 

 sense-organ the subradular organ. 

 The shell -pieces are traversed by 

 fine nerve-cords, which end in the 

 megalaesthetes and the micraes- 

 thetes. These are sensory epithelial 

 papillae, and in some forms the 

 megalaesthetes are modified into 

 eyes (Moseley), with a calcareous 

 cornea, a lens, and a pigmented 

 envelope. It has been asserted that 

 the ospliradia are multiple one at 

 the base of each ctenidium but it 

 is doubtful whether there are any 

 structures corresponding to the 

 ospliradia of other Gastropods. 

 Eidges of high epithelium contain- 

 ing sense-cells have been described 

 in the mantle-groove. 



The alimentary canal begins 

 with the mouth, which is placed on 

 a projection in front of the foot, the head or snout; it passes back 

 to the anus, which is median and posterior. The radula is well 

 developed, and its formula is (3 + 1) (2 + 1) 3 (1 + 2) (1 + 3). There 

 are two salivary glands with very short ducts. The oesophagus is 

 short, and is extended into a glandular pocket on each side. The 



FIG. 307. Central nervous system of 

 Chiton seen from the dorsal side. The 

 anterior and posterior ends of the 

 alimentary canal are represented dia- 

 grammatically in their relation to the 

 nervous system (modified from Pel- 

 seneer). I stomatogastric commissure 

 (buccal) ; II, labial commissure ; III 

 pedal cord ; IV anus ; V supra-rectal 

 pallial commissure ; VI pallial cord ; 

 VII visceral commissure; VIII sub- 

 radular commissure ; IX cerebral com- 

 missure ; X mouth. 



