1887.] 391 [Fewkes. 



Mnestra parasita, a Hydromedasa which lives parasitic on the pel- 

 agic mollusc, Phyllirhoe. We find here a modification in the 

 structure of Mnestra by the attachment, although we know but 

 little of the nature of that modification, while of the growth of the 

 medusa we know nothing. 



A most interesting instance of parasitism, and consequent modi- 

 fication among Medusae, is found in the problematical organism, 

 Polypodium. This undoubted hydroid is found parasitic in the ova 

 of the sturgeon while in the body of the fish. We have in Poly- 

 podium, as described by Ussow, a hydroid-like animal which 

 develops and drops buds which can be directly compared with 

 Medusae. These are not the only instances of parasitic Medusae 

 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 of that of related 

 adults with simple development. 



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

 mentioned instances of parasitism among Medusae 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, I believe, unique and 

 the first recorded example of a hydroid living attached to the 

 outside of a fish, and modified in structure by its life. It may 

 thus properly be called a new mode of life among Medusae. 



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

 years at the Newport Marine Laboratory we have taken several 

 specimens of the well-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 mak- 

 ing 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 regarded 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 super- 



