HYDROIDS. 65 



Melicertum, of Trichyra, and Lafoea, stand intermediate between them. 

 Resembling the youngest stages of the Campanularian Hydrarium we 

 have such forms as Clava and Rhyzogeton ; while the more branching 

 forms, Eudendrium and Bougainvillia, remind us already of somewhat 

 older stages. Lower still we must place Hydractinia, where the poly- 

 morphism of the individuals is an evident sign of inferiority, reminding 

 us of the free communities formerly separated from the Hydroids as 

 Siphonophores. From the close resemblance of the animal of the 

 Tabulata to such forms as Halocharis and the fresh-water Hydra, we 

 must consider them as an order, or perhaps only a suborder standing in 

 close relation to the Tubularians. Unsatisfactory as this may seem, 

 these few facts throw much light on our knowledge of the relations of 

 the Hydroids. Somewhat more satisfactory and more general results 

 can be obtained by comparing the young Medusas in their various 

 stages of growth. As I have already shown, in a short paper on the 

 order of appearance of the tentacles of Hydroid Medusas, the young, 

 when liberated, undergo great changes before arriving at their mature 

 condition ; and it requires a thorough knowledge of all these changes 

 to be able to recognize one and the same species in its various stages 

 of growth, and not to divide, as has been done so far, different species 

 by the number of tentacles, of marginal bodies, or the size of the ova- 

 ries. The main characteristic of the greater number of Tubularians, 

 when first liberated, is the totally different shape of the bell from that 

 of the adult. The bell is very deep, the number of tentacles is small 

 (Turritopsis, Bougainvillia, and Nemopsis) ; in the adult the shape of the 

 bell has become quite globular, the tentacles have increased in number, 

 the ovaries, which are generally absent or but slightly developed in the 

 young Medusas, have taken a development corresponding to their age. 

 Applying this to the standing of the different Tubularians, we should place 

 genera such as Clava and Eudendrium, in which the Medusas are always 

 sessile, lowest in their families ; next, the old genus Tubularia, such as 

 Tubularia proper, next Corymorpha, Hybocodon, then Ectopleura, where 

 we find the Medusa) losing almost entirely their embryonic character. 

 From these we pass to Sarsia, Syndictyon, Dipurena, Saphenia, Turns, 

 and Turritopsis. We then have families where the localization of the ten- 

 tacles, the position of the ovaries along the proboscis, and partly along 

 the chymiferous tubes, is a character of superiority, such as Dysmorphosa, 

 Lizzia, Bougainvillia, and Nemopsis, having a limited number of tentacles 

 placed at stated points along the circular tube. Closely allied to these 

 are such more Campanularian-like forms, as Melicertum, Ptychogenia, 

 and Staurophora, where the number of tentacles is large, but which 

 want the peculiar marginal bodies so characteristic of Campanularian 

 Medusas, and where the genital organs are intimately connected with 

 the digestive cavity. The young of these Medusas (Melicertum and 



NO. II. 9 



