716 



ZOOLOGY 



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

 ganglionic thickenings connected by a commissure, and furthi 

 back there may be a series of enlargements united by commissure 

 The pallial cords (/, /) are connected behind, above the rectui 

 by a commissure (p, c) which usually bears a median enlargemei 

 Sometimes a union takes place posteriorly between the cords 

 the two pairs. There are no eyes, or statocysts, or tentacl< 

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

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



-C0CC 



rect 



dia. 



cten. 



FIG. (502. ChretOderma nitidulum, longitudinal section, an. anus ; (>rn. brain ; cue. gla 

 ular caeca of mcsenteron ; cten. ctenidium ; dia. diaphragm separating off the posterior port 

 of the body ; mo. mouth ; peri, pericardium and heart ; rod. radula ; rect. rectum. (Af 

 Simroth.) 



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

 ring consisting of a thicker dorsal cerebral portion not differentiatec 

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

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

 apparatus. Two pairs of longitudinal nerve-cords, pedal and pallia/I 

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

 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 supr 

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

 its origin each pallial cord gives off a slender visceral commissu 



