29 
Agassiz (1, p. 87) as found (more than once) on a species of 
Pennella parasitic on Orthagoriscus mola ; later the same Hydroid 
was taken by Leidy (12, p. 165) on another Lernean Lernæonema 
procera parasitic on Odontaspis littoralis *). Hitherto this Hydroid. 
has only been observed growing on Lerneans on fishes; but it is 
very close to Eucope polygena , attacked to quite other substrata, 
and it seems questionable if this case is really different from that 
of Obelia geniculata , mentioned above. At all events the special 
i nte rest which we at first sight might attach to the examples quoted 
of associations between Hydroids and Fishes, and still more to those 
between Hydroids and Lerneans parasitic on Fishes, looses very 
much when we consider the structure of the Hydroids in question. 
None of these Hydroids, neither Stylactis minoi, nor Perigonimus 
« 
pugetensis, nor Eucope parasitica show any peculiar adaptation for 
their occurring on a living animal, not in the least any more than 
Obelia geniculata , usually found on quite other substrata; in no 
respect do they carry the stamp of being transformed owing to their 
peculiar habitation. Whatever the advantage may be for the one 
part of the association, the Fish or the Lernean, for the Hydroid 
it will possibly be that of getting an easier access to food supply; 
but the kind of food and the mode of grasping it I think must 
be the same as that of their nearest allies not found on living 
animals. 
With Ichthyocodium , Hydrichthys and Nudiclava the case seems 
to be different. In these Hydroids all the polypes have lost their 
tentacles; probably because they get their food in another way and 
take another kind of food than their nearest relatives 1 2 ). I think 
they depend in some way or other on the fish for food (the Para¬ 
sitic Copepod in the case of Ichthyocodium only serving as attach- 
1 ) Whether the “ Campanularia ” which Paul Mayer (15, p. 58) found 
growing in great numbers on the filaments of a Pennella (filosa?) 
parasitic on Xiphias gladius is identical with Eucope parasitica Ag. 
I am unable to decide. 
2 ) The absence of nematocysts in the polypes of Ichthyocodium seems 
to point in the same direction. 
