NEMOPSIS. 361 



Atlantic shores of North America. He knew nothing, however, of the trophosome, while he fell 

 into the mistake of supposing that the two clavate tentacles which occnr in every marginal 

 cluster of the medusa, carry each an ocellus on its summit, an error which was afterwards 

 rectified by M'Crady, to whom we are also indebted for having been the first to make us 

 acquainted with the trophosome of a congeneric species. 



A medusa, nearly allied to the Nemopsis described by Agassiz, was believed hy M'Crady to 

 be developed by budding from a free hydranth which he captured in the open sea, and he was 

 thus led to the belief that Nemopsis presented the very exceptional condition of possessing a free 

 floating trophosome. I feel, however, almost sure that the body here regarded by M'Crady as 

 the entire trophosome of Nemopsis is only a hydranth detached from its stem. We know that 

 this phenomenon is common in Tuhularia, in ■which we find the hydranth periodically detached 

 from the hydrocaulus, carrying with it its burden of gonophores, and continuing for some time 

 to live in this detached condition ; and I consider it highly probable that not only the free 

 hydranth of Nemopsis, but the supposed free trophosome of Stimpson's Acaulis, affords merely 

 another example of this phenomenon. 



M'Crady appears to entertain no doubt that the medusa to which he gives the name of 

 Nemopsis Gibbesii, and which he found free in the open sea, is derived from this free trophosome, 

 which, at the time of its capture, carried numerous medusa-buds ; but here again, though there 

 is no reason against the truth of this conclusion, the evidence is not complete, for it does not 

 seem that M'Crady traced directly the development of the medusa-buds into the adult Nemopsis- 

 medusa, but that he merely inferred the relation of the buds to the adult form from the observa- 

 tion of intermediate stages, which he discovered in the open sea, and through which he convinced 

 himself that the free adidt medusa could be connected with the buds detached from the tropho- 

 some. Coming, however, as this opinion does from so excellent and conscientious an observer as 

 M'Crady, we may, with all safety, accept it in anticipation of further confirmation. 



The presence of two different kinds of tentacles in each marginal cluster of the medusa is 

 an interesting feature in the genus ; and another very important one is the way in which the 

 generative lobes, which are at first confined to the walls of the manubrium, gradually extend 

 down the radiating canals towards their marginal termination.' 



The medusa of Nemojjsis is intimately allied to that of BougainvilUa, which it especially 

 resembles in the fact of each marginal bulb carrying a cluster of tentacles with ocelli, in that of 

 the mouth being provided with bifurcating tentacles, and in the great thickness of the umbrella 

 walls. 



See above, p. 63, where the significauce of this character is discussed. 



