THE CTENOPHORA. 



155 



seated, in many cases, a sac containing solid particles, like 

 one of the lithocysts of the medusiform Hydrozoa. I see no 

 reason to doubt that the rounded body is a ganglion and the 

 sac a rudimentary auditory organ. Bands which radiate 

 from the ganglion to the rows of paddles may be regarded 

 as nerves ; though they may contain other than nervous 

 structures.^ 



The ova and spermatozoa are developed in the lateral 

 walls of the longitudinal canals, which correspond with the 

 faces of the mesenteries in the Coralligena^ and the sexes 

 are usually united in the same individual. 



Fig. 31.— Diagram of Plevrohrac?iia.—a. month ; &. stomach : c. infuiidihuUim ; rf, 

 horizontal' canal; e. one of its' branches dividintr ajraiu av / into two branches 

 which open into the longitudinal canals, g <7, parallel with which the ciliated 

 area runs ; h. sac of the tentacle, ?, with one of its branches, k ; I, canal run- 

 ning by the side of the stomach; m, tentaculigerous canal; ?i n, canals opening 

 at the'aboral apertures, o, on each side of ^, the ganglion and lithocyst. 



1 Grant originally described a nervous gano'lionated ring, whence longitu- 

 dinal cords proceeded in Cydi'ppe {Pleurobrachla)^ but his observation has not 

 been verified by subsequent investigators. According to Milne-Edwards, fol- 

 lowed by others (among whom I must include myself), the nervous system 

 consists "of a ganglion, situated at the aboral ^ole of the body, whence nerves 

 radiate, the most conspicuous of which are eight cords which run down the 

 corresponding series of paddles ; and a sensory organ, having the characters 

 of an otolithic sac, is seated upon the ganglion.' Agassiz and Kolliker, on the 

 other hand, have denied that the appearances described (though they really 

 exist) are justly interpreted. And again, though the body, described as an 

 otolithic sac, undoubtedly exists in the position'indicated in all or most of the 

 Ctenophora^ the question has been raised whether it is an auditory or visual 

 organ. 



These problems have been recently reinvestigated with great care, and by 

 the aid of the refined methods of modern histology, by Dr. Elmer, whose de- 

 scription of the nervous system has already been quoted {supra^ p. 63). 



