366 Mr. J. W. Fewkes on a neio 



of Hydrichtliys it is interesting for us to consider those of the 

 attached Hydroid. If 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 de- 

 fended. 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 im- 

 portant 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 Tubularian Hydroid, as Coryne^ is a slender axis which 

 may be naked or encased in a chitinous tube, an enlargement 

 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 elsewhere on the stem, arise buds which may or may not 

 develop into Medusa3. The widest variations from such a 

 schematic type may be noticed among Hydroids. Our pur- 

 pose here is to compare Hydrichthys 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 Hydroids, and the Medusa-buds have been crowded 

 to the distal ends of these branches. Tentacles have disap- 

 peared on account of the parasitic nature of the life of the 

 Hydroid. 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 Avith 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 indi- 

 viduals in the Hydrichthys-colonyy it would seem as if there 

 ought to be a meaning for their simple structure as compared 

 with the typical Hydroid. The relation of the Medusa to 

 that of /SV«'6'/a- like genera would imply degeneration, not phy- 

 logenetic simplicity. Cannot we find in parasitism a cause 

 tor sucli a degradation ? 



Is the conclusion legitimate that these great differences be- 

 tween Hydrichthys and the fixed Hydroid closely related to it 

 are the result of its peculiar mode of life ? I believe it is. I 

 believe that the modification in the Hydroid Hydrichthys, the 



