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PROCEEDINGS OF THE 



[Dec, 1856, 



found in the species of this latter group, and not in those of the 

 former, the sub-generic distinction will be complete. 



Gregenbaur, in the paper above referred to, states that so far as 

 known the bydroids of Oceania are Syncorynidse. If, then, the 

 embryological development just described be really as I have 

 treated it, the development of Turritopsis nutricula, and not the 

 parasitic development of an inferior Medusa upon a structurally 

 superior one, (which is the only alternative,) we are not only 

 taught by it how small is the difference between alternate genera- 

 tion and direct metamorphosis, but we may be led to inquire in 

 other directions for the embryology of some genera, whose fixed 

 hydrse we have not been able to find, though they are evidently 

 related to others known to go through a regular alternation of gene- 

 ration of the ordinary type. In any case, the existence of at least 

 three instances wherein the hydra is directly metamorphosed into the 

 Medusa, viz: iEginopsis, Stenogaster, and the Medusan form in 

 Turritopsis, appears to destroy the universality of the so-called 

 alternation of generation as a character of Discophorae. 



POSTSCRIPT. 



This aceount of the development observed in the disk-cavity of 

 Turritopsis, was written before I was acquainted with the observa- 

 tions of Miiller and Kolliker upon the embryology, of /Rginopsis, 

 and Stenogaster in the Mediterranean. Though I have since pre- 

 fixed a notice of these observations and introduced some references 

 to them in the body of this Article, I have preferred not to alter its 

 general tone. I may, however, here observe, that the last stage of 

 the larva observed by myself, is by no means distantly allied to 

 Stenogaster com/planatus differing only, so far as I can determine 

 from a description without figure, in the number of the tentacula,. 

 and otolithic vesicles, and in the absence of the little knob on the 

 upper part of disk. This resemblance strengthens the probability 

 that my larva is only a parasite upon Turritopsis nutricula, though 

 certainly if this be the true construction, its history appears yet 

 more extraordinary, when we consider the manner in which it is 

 fed by its foster-mother. Yet the two forms of Medusas in whose 

 disk-cavities these similar larvae are developed, differ more from 

 each other than do the larvse. 



Before, however, venturing any positive conclusions as to the 

 proper solution of this interesting question, we must wait for the- 

 results of future researches. 



