2S87.] 393 [Fewkes. 



Hyd rich thy s it is interesting for us to consider those of the at- 

 tached hydroid. Jf our problem was to determine the relationship 

 of Hydrichthys from a study of the medusa alone, we could easily 

 conclude that it is a near relative of Sarsia. Such a conclusion 

 is, I believe, one which can be easily defended. When, however, 

 we come to compare the hydroid of Sarsia and the hydroid of 

 Hydrichthys, we find the greatest differences between the two. 

 These differences are so important that they have affected the 

 whole structure ; for a comparison of the two reveals the effect of 

 the peculiar mode of life in Hydrichthys. The typical structure, 

 or schema, of the itubularian hydroid, as Coryne, is a slender axis 

 which may be naked or encased in a chitinous tube, an enlarge- 

 ment at the free end, and a terminal mouth opening. This mouth 

 opening or the walls of the enlargement bear tentacles in rows 

 irregular or otherwise. Somewhere among these tentacles, or else- 

 where on the stem, arise buds which may or may not develop into 

 medusae. The widest variations from such a schematic type might 

 be noticed among hydroids. Our purpose here is to compare Hy- 

 drichthys with the so-called schema. 



In the case of the gonosome of Hydrichthys I suppose that the 

 stem of the schema remains, that the terminal mouth opening is 

 present, but that the enlargement of the axis has disappeared. 

 From the sides of the axis arise lateral branches as in some hy- 

 droids and the medusa buds have been crowded to the distal ends 

 of these branches. Tentacles have disappeared on account of the 

 parasitic nature of the life of the hj^droid. It is from this fact 

 that we find in Hydrichthys the schema of the ordinary tubularian 

 hydroid reduced to a simple sexual body or gonosome. 



In the homology of the "filiform bodies" of Hydrichthys the 

 reduction, as compared with the schema of a hydroid, has gone still 

 further on account of the parasitic life, and nothing remains but a 

 simple axis without appendages of any kind. 



If I am right in this homology of the two kinds of individuals 

 in the H} T driehthys colony, it would seem as if there ought to be a 

 meaning for their simple structure as compared with the typical hy- 

 droid. The relation of the medusa to that of Sarsia-like genera 

 would imply degeneration, not phylogenetic simplicity. Cannot 

 we find in parasitism a cause for such a degradation? 



Is the conclusion legitimate that these great differences between 

 Hydrichthys and the fixed hydroid closety related to it are the re- 



