28 
growing on living animals merely accidentally is well known. I may 
refer to Alcock (2, p. 207) who has collected a series of exarnples. 
From my own experience I might add a case, at first sight parallel 
to that of Ichthyocodium , namely that of Obelia geniculata, which 
I have seen flourishing on a Lernæa branchialis attached in the giils 
of the common cod; a similar case is mentioned by Sæmunds- 
son (21, p. 29). This Hydroid as well as those mentioned by 
Alcock normally grow on quite other substrata; by accident they 
may attach themselves to living animals, and they may occur on 
very different organisms. If, however, a Hydroid is quite regularly 
met with on the same animal — or a nearly related one — and 
is only found there, we may be sure that we have before us some 
kind of symbiosis, in most cases probably a form of commensalism. 
To decide whether the association involves a reciprocal advantage 
or is beneficial only to the one part is in most cases very difficult, 
and a matter of mere conjecture. Alcock also mentions a number 
of such regular combinations (1. c. p. 208), and he adds as a new 
case that of Stylactis (Podocoryne) rninoi , which he always found 
attached to the skin of the fish Minous inermis, while other species 
of Minous apparently were free of this Hydroid. Later the same 
has been observed at Japan (Franz and Stechow(8), Stechow 
20 a; PI. IV, Fig. 8). That after Heath (9) Minous inermis also 
may be found free of this Hydroid (Snyder is said by H. to have 
captured several specimens uninfested) in my opinion does not alter 
in any way the character of Stylactis rninoi as a symbiotic form; 
hitherto it has never been found on other substrata than the body 
of a Minous. A somewhat similar association is described by 
Heath (1. c.): of 37 specimens of the cottoid fish Hypsagonus 
quadricornis, captured in Puget Sound (Friday Harbor), 10 were 
coated with Perigonimus pugetensis, a new species related to P. 
vestitus Allm. 
As a “triple-association*' between a Hydroid and a Crustacean 
parasitic on a fish, which perhaps is a regular one, I might men- 
tion that of Eucope parasitica. This Hydroid is described by Al. 
