MUSEUM OF COMPARATIVE ZOOLOGY. 229 
The medusa with two opposite tentacles was raised into one with four 
(Plate V. Fig. 2), passing out of the stage resembling Stomatoca into one like 
Sarsia. 
The form of the bell and the arrangement of tubes is unchanged in the 
passage from the medusa with’two tentacles into one with four. The new 
tentacles form on the bell margin, half-way between those already formed. 
They arise near the junction of the radial and marginal canals. A1l the tenta- 
cles now grow to a great length, and the medusa, once very active, sinks to 
the bottom of the aquarium. Its motion is from now on more sluggish than 
before, either from exhaustion or habit. I was unable to raise them into 
meduse with more than four tentacles. 
The affinities of Hydrichthys would not be difficult to make out if we were 
to deal with the medusa alone. So close are the resemblances with such gen- 
era as Sarsia, Ectopleura, and other allied Tubularians, that there would have 
been no doubt in my mind, if I had the medusa alone to deal with, that Hy- 
drichthys is a close ally of these genera. It is the form of the hydroid which 
complicates the problem in regard to the affinities of the parasite, for, so far as 
the hydroids of the Tubularians allied to Sarsia are concerned, there are none 
which have any resemblance to the hydroid of Hydrichthys. 
If we approach the study of Hydrichthys from the hydroid side, remember- 
ing the undoubted affinities of the medusa, it seems to me that we must regard 
the modifications in its structure and its polymorphism as due to the attach- 
ment to the walls of its host, the fish. We know, of course, too little of the 
other possible habitats of this strange hydroid to declare that it is never found 
in any other place, but the general structure of its body would seem to point 
to a special modification of its structure brought about by its parasitic life. 
The peculiarities of structure which separate Hydrichthys from other allied 
Tubularian hydroids are the total absence of tentacles, combined with a poly- 
morphism in which there are two kinds of individuals already described, 
viz. botryoidal gonosomes and filiform hydranths (?).* 
In all Tubularian hydroids there are tentacles of some kind or other near a 
mouth opening. In Tubularia, for instance, we have circles of tentacles ar- 
ranged about a mouth, and from the intertentacular regions or intervals on the 
head hang down grape-like clusters of gonophores. Suppose, for purposes of 
comparison with Hydrichthys, that in Tubularia the chitinous sheath of the 
single hydroid is absent, the tentacles reduced to nothing or absent, and the 
* It might be supposed that the second of these are simply the main stem of 
the gonosomes, stripped of lateral branches with medusa buds. The differences 
in the structure of the two show the error of such a supposition. It might be 
objected to my interpretation that there are two kinds of individuals in Hydrich- 
thys, on the ground that the filiform bodies are undeveloped gonosomes. That 
objection is also believed to be poorly supported, for young gonosomes differ 
even as markedly as the adults from the filiform bodies. The designation of the 
filiform bodies as hydranths is simply conjectural. 
