﻿April, 1857.] 



ELLIOTT SOCIETY. 



109 



we find upon close study, should be only extremes of a single 

 family group. Yet since we are in reality acquainted only with 

 a comparatively small number of genera among Medusae, and those 

 of European and American seas only, we certainly are not yet fully 

 prepared to define families by structural limits, a mode of classifi- 

 cation which requires the most extensive comparison of generic 

 forms. If, however, we call in the aid of Embryology, in connec- 

 tion with such knowledge of structure as we have, we may obtain 

 views, it appears to me, which even should they not prove abso- 

 lutely correct in the end, will still be suggestive and lead in time 

 to the desired natural classification. 



For taking it for granted as I do, that the Hydroid polyps and 

 the Gymnopthalmata of Forbes constitute a single natural order, 

 and knowing that in a considerable number of instances, as for 

 example, in Clava, Hydractinia and some Tubularise, the Medusa 

 never forsakes its polyp-stalk until death,* we see that we cannot 

 omit in classification a careful consideration of the peculiarities of 

 the Hydroids, which are so complex in their gradations that Eh- 

 renberg formed two distinct groups upon the basis of the Linnean 

 genera, Tubularia and Sertularia. These two groups, I believe, 

 have been generally acknowledged by authors since the publica- 

 tion of Ehrenberg's work.f Not having this work, I quote the 

 characters of these two groups from Johnston's British Zoophytes. 

 They are distinguished by their different characteristic methods 

 of producing reproductive buds, (ovisacs and bulbules, Johnston.) 

 The Tubularina include those Hydroids which produce naked 

 bulbules — that is, those in which the medusa-buds, while attaining 

 sufficient growth to become free, are not protected by a horny 

 theca. The second, the Sertularina, are those which have 

 what are called ovisacs — horny thecas or cases, bell-shaped usu- 

 ally, and closed at their free open extremities by a polyp- 



* This occurs even in genera, where the rule is, that Medusae are free. For 

 example, I once had the opportunity of observing a fine specimen of Sarsia 

 mirabilis (Agassiz,) which is one of the most abundant free species in Boston 

 Harbor, wither on its stalk, never severing its connection with its Coryne, 

 t hough in every respect, except ocelli, which were absent, it resembled the per- 

 fect animal. It unfolded its four long tentacula, exhibited strong rythmical con. 

 tractions of the disk, and behaved in all respects like a perfect Medusa except 

 that it never became free. 



f Corall. der Roth. Meer. (1834.) Lamouroux had previously subdivided these 

 genera, at the same time recognizing their affinity to each other more clearly 

 than Lamarck. 



