Mode of Life among Medusoi. 365 



fungus of Seriola and Sajij'olepiia, that at first I regarded 

 the former as a fungoid growth. The colour of the supposed 

 fungus of SerioJa was, however, reddish and yellow ; and, 

 although I have since learned that superficial fungoid 

 growths of this colour sometimes exist on fishes, at the time 

 when Seriola was captured I was ignorant of this fact ; the red 

 colour led me to doubt its fungoid affinities. A glance at the 

 supposed fungus through a small lens easily dispelled my 

 error and showed me that I had a new and unique case of a 

 parasitic Hydroid, It is to the peculiarities in structure of 

 this animal and the Medusa which was raised from it that I 

 wish to call attention in the present paper. 



As the genus of Hydroids which shows this curious mode 

 of life is new, it will be necessary to assign it a name, and I 

 suggest that of Hydi-ichthys tnirus, as expressing one phase 

 at least of the curious life which it leads *. 



The majority of genera of Hydromedusfe have ordinarily 

 two stages of growth, one of which is called the Hydroid and 

 the other the Medusa-stage. Tlie latter is a Medasiform 

 zooid of the former. Let us consider each of these stages in 

 Hydrichthys. 



Hydroid. — The Hydroid of HydrichtJiys consists of sexual 

 and asexual individuals, both of which arise from a flat plate 

 of brandling tubes which is fastened to the sides of the body 

 of the fish. The sexual individuals may be called the gono- 

 somes, 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 resembling minute grape- 

 like bodies. These grape-like bodies are Medusse in all stages 

 of growth. The filiform individuals are simple flask-shaped 

 bodies, without tentacles and wnth terminal mouths f. 



iVo circle of tentacles about a mouth-opening teas detected 

 either in the gonosomes or the filiform bodies. This is a sig- 

 nificant want, since, with the exception of Protohydra, Micro- 

 hydra, and the secondary zooids of certain Alcyonians, ten- 

 tacles of some kind are found near a mouth or in relation to 

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



Medusa. — The gonophore of Hydrichthys has a 6^ar*"i.a-like 

 bell and manubrium, four radial tubes, four tentacles without 

 appendages, as already elsewhere described by me \. 



In the light of what we know of the affinities of the Medusa 



* An accurate diagnosis with figures will be found in my paper "On 

 certain Medusje from New England," Bull. Mas. Coinp. Zool. xiii. no. 7. 



t Souiewliatlike the spiral zooids in Fcriyonimus except this particular. 



\ Bull. Mus. Comp. Zool. xiii. no. 7. 

 Ann. & Mag. N. Hist. Ser. 6. Vol. i. 25 



