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PROCEEDINGS OF THE 



[April, 1857. 



known through the researches of Huxley, Vogt, Kolliker and 

 Gegenbaur, under the name Chrysomitra, and has already been 

 referred to the neighborhood of Sarsiadas, Cladonemidse and 

 Oceanidse, by Gegenbaur, (Zeitsch loc. cit.) It has, however, 

 some very remarkable peculiarities which I shall notice more par- 

 ticularly when I come to describe our own species of Willsia. 

 Whether the Physophoridse which seem to be communities of 

 Medusas rather than of Hydroid Polyps, should constitute a sepa- 

 rate subdivision in the group of Tubularina on account of the 

 wonderful degree of specialization among the individuals of a 

 single community, I do not undertake to decide, since, unfortu- 

 nately, my observations on our own species, in spite of my efforts 

 to the contrary, have hitherto been too limited to enable me to add 

 anything to our knowledge of the group. It should, however, it 

 seems to me, be borne in mind that the social insects whose 

 classes of individuals are also differentiated upon an embryo- 

 logical basis, just as is the case with Siphonophoras, do not con- 

 stitute a single group, but are distributed among different orders 

 and different families in the same order. This analogy, though 

 rather a remote one, should, it seems to me, be allowed some 

 weight in guiding the investigation. The Velellidae, on the other 

 hand, seem to be floating polypidoms, with only one class of Me- 

 dusas, i. e., those with reproductive organs. The Physophoridse 

 are, to a certain extent, comparable to the budding Sarsias and 

 Lizzias, and the Velellidas to Tubularia, Hydractinia, &c. 



The singular group of Aeginidas comes up next for considera- 

 tion. I have already mentioned that a series of gradations can be 

 established between the genera Cunina, Aegina, Aegineta and 

 Aeginopsis and Polyxenia, and the genus Eucope, the Medusa form 

 of Campanularia. First, Equorea by its digestive cavity and 

 mouth, the correspondence of other characters appears to be a 

 form nearer to the Aeginidas than to the Eucopidas. Next comes 

 Rhopalonema and Stomobrachium, in which the number of tubes 

 is already reduced, and the sexual organs are found in the two 

 forms, the circular and elongated, which are common in the sub- 

 divisions of Eucope. 



While, therefore, there is a structural gradation between the two 

 groups, we find a discrepance in their embryological .history. In 

 Campanularia, as is well known, there is a regular individualized 

 metamorphosis or alternate generation. But among the Aeginidas, 

 where the embryology is known in at least three instances, 



