igoy.] Records of the Indian Museum. 285 



pointed fir cone ; it is more opaque than the female one, and con- 

 sequently can only be properly studied in serial sections. Figures 

 6 to 8 show three of a series of such sections. The endodermal 

 contents are more complex than those of the female form and do 

 not seem quite the same in ever^^ case, but like that form there is 

 a spadix which shows a more or less C-shaped figure in longitudinal 

 section. No male gonophore was met with in a ripe condition ; 

 they mostly contained spermatoblasts. 



Theoretical considerations — 



In spite of the anomalous structure of the hydranth, this 

 genus should, I think, find a place among the gymnoblastic 

 hydroids, and a comparison with two hydroid genera, Hydrichthys 

 niirus and Stylactis minoi, which are also parasitic on fish, leads 

 to some interesting conclusions. 



Stylactis minoi was described by Alcock in 1892 (5) and has 

 been since found several times in Indian seas, always attached to 

 the skin of the small rock perch Minous incrmis. It is a typical 

 hydroid in every way. 



The peculiar form Hydrichthys mirus discovered in 1887 by 

 Fewkes growing to the carangoid fish Seriola zonata at Newport, 

 U.S.A., cannot be called a typical hydroid. It resembles the 

 present genus very closely in some respects, in others it differs 

 widely from it. HydrichtJiys is described as follows by its dis- 

 coverer : — 



'' The base of attachment to the fish is a flat, thin plate 

 with ramifying tubes, by means of which the colony is fastened to 

 the fish, and upon it separate clusters of sexual bodies (gonosomes) 

 and filiform structures (hydranths ?) are united together." 



The author compares this basal plate to that of Hydractinea, 

 without the chitinous projections, and it is obviously very like 

 that of the genus described here. Hydrichthys, however, has long 

 arborescent gonosomes to which medusae in all stages of develop- 

 ment are attached. The fish, with its parasite, was kept alive in 

 an aquarium and '' thousands of these medusae were raised. " 

 The medusae swim freely, and each has four tentacles. The 

 generative organs are therefore totally different from those of the 

 new genus, in which these organs are represented by a few closed 

 sporosacs, sessile on the basal plate. Turning now to the hydranth, 

 the comparison between the two forms is of such interest that it 

 seems well to quote Fewkes' s account in extenso, especially as 

 the nature of the hydranth of Hydrichthys is regarded somewhat 

 doubtfully by that author : — 



'■ In addition to the botryoidal clusters of gonosomes there 

 also arise from the basal plate by which the colony is fastened to 

 the fish, long, flask-shaped bodies, recalling in their external form 

 the tasters of the Siphonophores. These bodies, like the gonosomes, 

 arise from the upper walls of the basal plate of tubes attached to 

 the body of the fish. I^ike the gonosomes they are numerous in 

 the hydroid colony. Tiie filiform bodies are elongated flask-shaped 



