208 THE AMERICAN MONTHLY [November, 



droid-like animal which develops and drops buds which can be directly com- 

 pared with Medusae. These are not the only instances of parasitic MedusjE 

 thus far recorded, but they are typical and useful for comparisons. None of 

 them are as valuable as they might be in estimating the amount of change in 

 anatomy which has resulted, since we are either ignorant of their whole life- 

 history or that of related adults with simple developinent. 



It is with the greatest pleasure that I am able to add to the above-men- 

 tioned instances of parasitism among Meduste another of most extraordinary 

 character. This instance is peculiarly adapted for the study of the effect of 

 parasitism in modifying the Medusan structure, as its close allies are -well 

 known and comparisons with them can be easily made. This instance is the 

 first recorded example of a hj-droid living attached to the outside of a fish, 

 and modified in structure by its life. 



In the pelagic fishing which has been carried on for the last ten years at 

 the Newport Marine Laboratory w^e have taken several specimens of thev\^ell- 

 known fish, Seriola zonata^ Cuv. This fish is a close ally of the ordinary 

 ' pilot fish ' and is often seen in calm weather swimming near the surface of 

 the sea. Three of these fishes were found in company last summer, and 

 upon the side, near the anal fin, of one of these, curious appendages were 

 noticed which had never been observed before. On capturing the fish and 

 making a superficial examination of the attachment, I was reminded of an 

 attached fungus growth. Everyone is familiar with the growth on fishes of the 

 fungus Saprolegnia, and the resemblance seemed so great, except in color, 

 between the supposed fungus of Seriola and Saprolegnia that at first I re- 

 garded the former as a fungoid growth. The color of the supposed fungus 

 of Seriola was, however, reddish and yellow ; and, although I have since 

 learned that superficial fungoid growths of this color sometimes exist on 

 fishes, at the time when Seriola was captured I was ignorant of this fact ; the 

 red color led me to doubt its fungoid affinities. A glance at the supposed 

 fungus through a small lens showed me that I had a new and unique case of 

 a parasitic hydroid. 



As the genus of hydroid which shows this curious mode of life is new, it 

 will be necessary to assign it a name, and I suggest that of Hydrichthys 

 7nirus as expressing one phase at least of the curious life which it leads. The 

 majority of genera of Hydromedusae have ordinarily two stages of growth, 

 one of which is called the hydroid and the other the medusa stage. The 

 latter is a medusa-form zooid of the former. Let us consider each of these 

 stages. 



Hydroid. — The hvdroid of Hydrichthys consists of sexual and asexual in- 

 dividuals, both of which arise from a flat plate of branching tubes which is 

 fastened to the sides of the body of the fish. The sexual individuals may be 

 called the gonosomes, the asexual the filiform bodies. 



The gonosomes consist of a simple contractile, highly sensitive axis, upon 

 the sides of which are borne lateral branches with terminal clusters resem- 

 bling minute grape-like bodies. These grape-like bodies are medusae in all 

 stages of growth. The filiform individuals are simple, flask-shaped bodies, 

 without tentacles, and with terminal mouths. 



No circle of teritacles about a motith opening ivas detected either in the 

 gonosomes or the filiform bodies. This is a significant loss, since, with few 

 exceptions, tentacles of some kind are found near a mouth or in relation to 

 the oral opening of most of the fixed hydroids or polyps. 



JMedusa. — The gonophore of Hydrichthys has a Sarsia-like bell and ma- 

 nubrium, four i-adial tubes, four tentacles without appendages. 



In the light of what we know of the affinities of the medusa of Hydrichthys 

 it is interesting for us to consider those of the attached hydroid. If our 



