XII 



PHYLUM MOLLUSCA 



667 



peri 



These are connected together by an oesophageal ring, thickened 

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

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

 ganglia. The pedal 

 cords (v.v) may pre- 

 sent in front a pair 

 of ganglionic thick- 

 connected 

 imissure, 

 [er back 

 ry be a 



F;G. 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 ; 

 ent. enteric cavity ; ft. foot ; gon. gonad ; lid. head-lobes ; 

 lit. heart ; liv. liver ; mo. mouth ; neph. nephridium ; pen. 

 pericardial cavity. (From Simroth, after Haller.) 



CO 



enlarge- 

 nited by 

 res. The 



palliaWords (LI) 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 elevc^tion in 

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



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

 ring consisting of a thicker dorsal cerebral portion not differentiated 

 into ganglia, and a thinner ventral l)uccal commissure. The cerebral 

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



FIG. 567. Nervous system of Amphineura, A, Proncomenia ; .-\B, Neomenia : C, Chcetoderma ', 

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

 commissure or ring, with ganglia ; v, v, pedal cord's. (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 



