﻿OF THE ACALEPHiE OF NORTH AMERICA. 365 



fully the arrangement of the contractile fibres throughout the body. The general ar- 

 rangement is very probably the same as in Pleurobrachia ; for in some dying specimens, 

 I have seen, at some distance from the central black speck, eight points between the 

 ambulacra, alternating with them, arranged, as in Pleurobrachia, in the form of an oval, 

 which indicate probably the presence of as many vertical bundles of fibres running from 

 the upper summit downwards, and regulating the movements of the lobes jointly with 

 circular fibres, which are more easily detected, and occur in great numbers grouped 

 together in bunches along the sides of the vertical rows of locomotive fringes in the 

 space intervening between their combs, and extending brush-like horizontally into 

 the substance, though diverging in each bundle. These fibres seem more powerful, 

 and, at all events, far more distinct, than the vertical muscles, which I have never 

 been able to trace in continuous rows. Some means might perhaps be found to 

 preserve their bodies in such a way as to bring out the muscular fibres. It would, 

 indeed, be very interesting to study their arrangement, especially with reference to 

 the motions of the large lobes. 



Though I have watched specimens of this species at short intervals through six 

 successive months, from December to June, I have never succeeded in discovering 

 the sexual system, not even in the most rudimentary state. Should it, however, 

 be found to follow in its development the course of the ambulacra! tubes, as reported 

 by Wild, this fact would go far to show the homology, to which I have above alluded, 

 between the ambulacral tubes of Beroid Medusae, and the vertical chymiferous tubes 

 of Naked-eyed Medusae. The circumstance of my failing to trace the reproductive 

 system after so long a search, may show how great difficulty these investigations 

 are attended with, and how much remains to be done before the whole history of 

 these animals is satisfactorily made out. 



Of their embryonic development nothing at all is known ; but from the character 

 of the black specks, I would repeat what I have said of Pleurobrachia, that I sup- 

 pose them to be developed from Hydroid Polypi. 



I have observed a third Beroid Medusa on the shores of Massachusetts, in Edgar- 

 town harbour and at Nahant, belonging to the genus Idya, but it was under such cir- 

 cumstances that I could neither examine it carefully, nor have drawings of it made to 

 my satisfaction. I may say, however, that this species, at least in the condition in 

 which it was observed, is much smaller than any of those described before. I regret the 

 more to have been prevented from making a minute investigation of it, as that genus 

 is particularly remarkable for the lateral ramifications of the ambulacral tubes, and it 

 would be very important to compare these ramifications with the radiating ramifica- 

 tions of the chymiferous tubes in Discoid Medusae. 



