288 R. E. Ll.OYD : A nciv genus of Hydroids. [VOL. 1, 



tentacles and a hypostome^ and clearly catches its food like other 

 h3^droids. The hydrophyton is in the form of a creeping stolon 

 which may almost entirely cover the fish. These differences point 

 to a different mode of life from the other parasitic forms. An 

 explanation of these differences seems to be found in the different 

 nature of the fish. Minous inermis has been found many times in 

 the Bay of Bengal in company with such teleostean genera as 

 Uranoscopus , Platycephalus , Lophius, Pterois, which are essentially 

 bottom fish : whereas the fish hosts of the other two genera 

 under discussion were both captured in the tow net. The extent 

 to which the Minous is coated with the hydroid growth, caused 

 its discoverer to hold the opinion that the hydroid must benefit 

 the fish by concealing it to some extent. On this assumption, we 

 can imagine the Minous remaining still for considerable periods 

 of time during which the Stylactis could pursue its vocation of 

 catching prey, in the fashion of other h^^droids which are attached 

 to rocks. 



We see, therefore, that whereas the modes of life of Hydrich- 

 thys and Nudiclava seem essentially similar, they both differ 

 considerably in this respect from Stylactis minoi : although all three 

 forms appear to be hydroids parasitic on small teleostean fish. 



Affinities — • 



Comparison with other more normal hydroid types has not 

 led to any definite conclusions as to which particular type this new 

 genus may have been derived from. It undoubtedly resembles the 

 abnormal genus Hydrichthys in some ways, in the structure of the 

 basal plate and the absence of tentacles, and in its mode of life 

 generally. Here the similarity stops, and the two genera are sepa- 

 rated by the great differences in the gonophores, and in the internal 

 structure of the hydranth, which in Hydrichthys is quite of the 

 usual hydroid type. The conclusion arrived at is that the 

 similarities have been acquired in adaptation to the circumstances 

 of the peculiar life which are alike in both cases ; while the dif- 

 ferences are due to the fact that the ancestors of both forms which 

 took to this parasitic life were essentiall}' different, especially as 

 regards the nature of the gonophores. The genus Nudiclava has, 

 however become more specialised than Hydrichthys, as the result 

 of this mode of life. 



Stylactis minoi presents a third example of a hydroid, which 

 has scarcely been modified at all by its association with a fish. 

 Being attached to the skin of a sluggish rock-haunting species, it 

 is capable of obtaining food in the same manner as most other 

 hydroids. Consequently its structure has not been modified. 



Definition of the genus — 



The hydrophyton is a compact plate-like structure composed 

 of an irregular labyrinthine coenosarc with very poorly developed 

 perisarc. 



