62 



THE AMPHINEURA 



corners of the pericardial cavity, run forward under the floor of 

 the pericardium, and then turn 'outwards and backwards to run 

 back to their respective apertures right and left of the anus. 

 Their thin walls are lined by a ciliated epithelium, and there are 

 no accessory generative organs. 



In the nervous system there are two intimately fused cerebral 



r 



FlO. 43. 



Chaetoderma nitidnlum. A, median sagittal section ; B, sagittal section of the posterior 

 extremity ; C, sagittal section of the anterior extremity, a, anus ; br, retractor muscle of the 

 branchiae ; c.g, cerebral ganglion ; d.t, digestive tract ; g, gill ; go, gonad ; h, heart ; t, intestine ; 

 k, kidney ; I, liver ; m, mouth ; me, " mesothorax " ; p.c, pallial suprarectal commissure ; p.d, 

 pericardial duct ; pe, pericardium ; pe.c, pedal commissures ; pr, " prothorax " ; r, radula ; .c, 

 Hublingual commissure. (After Wiren.). 



ganglia bearing accessory lobes. Each ganglion gives rise to two 

 longitudinal nerve -cords, the ventral or pedal cord being more 

 slender than the dorsal or pallial cord. In the anterior part of 

 their course the pedal and pallial cords of either side run parallel 

 and adjacent to one another, but in the posterior region of the 

 body they are fused together, as in Paramenia, and the two pallio- 

 pedal cords thus formed are united dorsad of -the rectum by a 



