?16 



ZOOLOGY 



SECT. 



ganglia. The pedal cords (v.v) may present in front a pair of 

 ganglionic ohickenings connected by a commissure, and further 

 back there may be a series of enlargements united by commissures. 

 The pallial cords {I, f) are connected behind, above the rectum, 

 bj' 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, or statocysts, or 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. 



hrn 



Cbcn, 



perv 



Fig. 002.— 'ChsetOderma nitidulum, longitudinal section, an. anus ; hrn. brain ; cue. gland- 

 ular ciecaof mcsenteron ; cten. ctonidium ; dla. diaphnigm separating off the posterior portion 

 of the body ; mo. mouth ; 'qhtI. pericardium and heart ; roAl. radula ; met. rectum. (After 

 Simroth.) 



In the Placophora (Fig. 604, D) there is an oesophageal nerve- 

 ring consisting of a thicker dorsal cerebral portion not differentiated 

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

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

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

 are given off posteriorly : the former, from which arise nerves to 

 the foot, are joined by numerous commissures passing beneath 

 the enteric canal ; the latter, which send off nerves chiefly 

 to the mantle and the ctenidia, are united together by a supra- 

 rectal commissure at the posterior end of the body. Just behind 

 its origin each pallial cord gives off a slender visceral comviissure, 



