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. All 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 

 medusae 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. 



11" 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. 



