MUSEUM OF COMPARATIVE ZOOLOGY. , 225 



Fig. 1) and near the anal opening of this fish a patch of reddish-colored bodies 

 was noticed. This patch was at first supposed to be a fungoid growth from a 

 wound or abrasion of the body. A laore careful examination of the supposed 

 fungus showed me my error, and revealed the fact that it was an attached ani- 

 mal with true hydroid aifinities. The fish with the attached hydroid was 

 kept alive in an aquarium for some time, and from the hydroid many me- 

 dusae (Plate V. Figs. 1-3) of interesting relationship developed. Thousands 

 of these medusae were raised, and the general characters of their structure and 

 external anatomy studied. They seem to be hardy in their younger stages, 

 but it is doubtful whether I have raised them into adults.* The form of the 

 medusa is quite different from that of any known genus thus far found at 

 Newport, but not unlike in general affinities certain well-known tubularian 

 genera commonly found there in surface fishing, f 



The exceptional, and it is believed unique, condition in Hydrichthys is the 

 character of the hydroid, and the imusual feature its attachment to the sides of 

 the body of the fish as a parasite or commensalist-J The polymorphic structure 

 of the hydroid is quite different from that of any known hydrozoon. 



The modifications in the anatomy of the hydroid are believed to have been 

 in part due to its attachment' to its host. This supposition, if it is well 

 founded, and the additional fact that Hydrichthys has never been foimd in 

 any other habitat or attached anywhere else than to the body of an osseous 

 fish, may mean that we have in this genus a case of parasitism, or possibly 

 commensalism, and that this condition has rendered functionally useless or 

 modified the form of certain structures commonly present in other hydroids, 

 while it has increased in relative size and possibly importance other organs, 

 especially those concerned in reproduction and the dissemination of the young. 

 Hydrichthys, looked at in this light, presents us with one of the most interest- 

 ing conditions of hydroid life which has yet been recorded. 



It was impossible to determine how much nourishment the hydroid Hy- 

 drichthys draws from the fish upon which it lives through the network of 

 tubes from which the gonosomes and filiform bodies arise. The absence of 

 tentacles, or organs the function of which is the capture of food, would seem to 

 deprive Hydrichthys of those means of capturing and drawing food to the 

 mouth which are almost universal among fixed hydroids. Possibly in its 

 parasitic life the hydroid obtains its sustenance from the fish on the sides of 

 which it lives. 



The question whether the fish ultimately succumbs to the parasite is an 

 interesting one, but one which cannot be definitely answered at present. The 

 only specimen of Seriola captured which was infested by the hydroid appeared 

 to be well and healthy, and lived for a considerable time without exhibiting 



* The oldest medusa raised from the parasitic Hydrichthys has four tentacles, 

 t Sarsia mirahiUs and Ectopleura ochracca. 



X From my limited knowledge of Hydrichthys v^e are not justified in consider- 

 ing it a commensalist. 



vor,. XIII. — NO. 7. 16 



