HYDROIDyE. G7 



to the Medusae of some of our Tubularians, that McCrady even proposed 

 to separate the Velellidoe from the Siphonophorse, and to place them 

 next the Tubularians ; the sexual Medusae, also, of several of these free 

 Hydroids resemble very closely other Medusa?, as those of Hybocodon, 

 Corymorpha, and the like. When we add to this the strong argument 

 derived from the homology of the development of the Hydroids, whether 

 free or floating, as is shown hereafter from Nanomia, we can have but 

 little hesitation in acknowledging the value of the order of Hydroids as 

 first limited by Professor Agassiz, and the return, as proposed by him, 

 to the old subdivisions of Eschscholtz, the great master in the classifica- 

 tion of the Acalepha?, whose views seem to stand out brighter with 

 every fresh investigation. For certainly the subdivision by Leuckart 

 of the Siphonophorae into two suborders, and the uniting of Physalia 

 and Porpita and the like into one order with Agalma and its allies, is a 

 disregard of the true value of the ordinal characters which are to be 

 found in the combination of the float with the rest of the community, 

 such as we find developed in the three great phases of embryonic 

 growth of a Physophore. (See Nanomia.) As to the true position of 

 the different orders of the old group of Siphonophorae among the 

 Hydroids, we cannot fail to consider them as lowest in the series ; they 

 form communities, the different individuals of which never attain the 

 high degree of complication and the individuality so characteristic of 

 the Campanularian Medusae, and they must therefore rank lowest, next 

 to Hydractinia and the like, which form the connecting link between 

 them and the truly fixed Hydroids. 



In the limitation of the families of Hydroids, it is very difficult to 

 draw any line of demarcation, whenever we attempt to separate, as dis- 

 tinct families, those Medusae which are always sessile, from those which 

 lead an independent existence. The close affinity existing between the 

 Hydroids of genera in which we have free and sessile Medusae, seems to 

 preclude the idea of separating them as distinct families, notwithstand- 

 ing the great difference of form between the adult Medusae. As our 

 knowledge of the embryology of Hydroids becomes more extended, 

 cases occur more frequently in which Hydroids, so closely allied that it 

 is difficult to distinguish them generically, unless it be in the breeding 

 season, produce Medusae which are either sessile, or lead an independent 

 existence ; for instance, the many species of Campanularians closely allied 

 to Laomedea, the Tubularians of the genus Tubularia, and the different 

 species formerly referred to Eudendrium. We must combine, as far as 

 we are able from existing information, our knowledge of the Medusa and 

 of the Hydrarium ; this seems the only rational method, and one which 

 has already lead those who have adopted it to very important relations 

 of the true affinities of Acalephae. This view of the proper method to 

 be followed in the classification of Hydroids has been frequently em- 



