XII 



PHYLUM MOLLUSCA 



667 



per 



h-d 



These are connected together by an cesophageal ring, thickened 



dorsally into a single or double cerebral ganglion ; and in front of 



this is a second, more slender stomatogastric nerve-ring with small 



ganglia. The pedal 



cords (v.v) may pre- ht 



sent in front a pair 



of ganglionic thick- 



enings connected 



by a commissure, 



and further back 



there may be a 



series of enlarge- 



ments united by 



commissures. The 



pallial cords (/./) are 



connected behind, 



above the rectum, 



by a commissure (p.c) which usually bears a median enlargement. 



Sometimes a union takes place posteriorly between the cords of 



the two pairs. There are no eyes, nor otocysts, nor tentacles. 



Some have a sensory frontal lobe and a sensory pit or elevation in 



the middle line of the dorsal surface near the posterior end. 



In the Placophora (Fig. 567, D) there is an cesophageal nerve- 

 ring consisting of a thicker dorsal cerebral portion not differentiated 

 into ganglia, and a thinner ventral Tniccal commissure. The cerebral 

 part sends off nerves to the labial palps, the lips, and the buccal 



FJG. 566. Diagrammatic longitudinal section of Chiton, 

 specially intended to show the relations of the parts of the 

 ccelome, which are bordered with a thick line. an. anus ; 

 nit. enteric cavity; ft. foot; (/on. goiiad ; /'. head-lobes; 

 /(/.heart; In: liver ; TOO. mouth; neph. iiephridium ; '[.ri. 

 pericardial cavity. (From Simroth, after Haller.) 



FIG. 567. Nervous system of Amphineura. A, Pronu>,,, min ; B, Kcommia; C, Cltn/n,/, .,-,, i ; 

 It, Chiton, c, cerebral ganglia ; /, I, pallial cords ; pc, posterior commissure ; s, stomatogastric 

 commissure or ring, with ganglia ; c, c, pedal cords. (From the Cambridge Natural History, 

 after Hubrecht.) 



apparatus. Two pairs of longitudinal nerve cords, pedal and pallial, 

 are given off posteriorly. The former, which give off nerves to 

 the foot, are joined by numerous commissures passing beneath 

 the enteric canal. The latter, which send off nerves chiefly 



