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 more careful examination of the supposed 
fungus showed me my error, and revealed the fact that it was an attached ani- 
mal with true hydroid affinities. The fish with the attached hydroid was’ 
kept alive in an aquarium for some time, and from the hydroid many me- 
duse (Plate V. Figs. 1-3) of interesting relationship developed. Thousands 
of these medusz 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.t 
The exceptional, and it is believed unique, condition in Hydrichthys is the 
character of the hydroid, and the unusual feature its attachment to the sides of 
the body of the fish as a parasite or commensalist.{ 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 found 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. 
+ Sarsia mirabilis and Ectopleura ochracea. 
+ From my limited knowledge of Hydrichthys we are not justified in consider- 
ing it a commensalist. 
VOL. XIII. — NO. 7. 15 
