678 DAVID HILT TENNENT. 
from the “ Hecht” show a form exactly identical with the 
immature Gasterostoma which I have found in Tylosurus. In 
his description of the older stages he described a complicated 
structure of the anterior end of the body. Ziegler (7) has 
shown where he was in error in this description. 
Even a most casual comparison of the form which I have 
studied, with Ziegler’s figures of G. fimbriatum impresses 
me with the great similarity between the two forms, and 
when one enters into a more careful comparison, comparing 
section with section, the impression deepens to a conviction 
that the two forms are identical. But G. fimbriatum is the 
parasite of fresh-water fishes, while the Gasterostomum of 
the present paper finds its host among marine forms. This 
fact might be regarded as sufficient warrant for placing the 
forms in separate species. An additional reason for such a 
classification might be found in the fact that B. Polymor- 
phus, the larval form in the one case and B. haimeanus, 
the larval form in the other, are somewhat different, chiefly 
in the length and shape of the tails. 
As I have gone over the literature, comparing the descrip- 
tions and the figures published by various investigators, and 
bearing in mind the results of my own observations, I have 
been gradually forced to the conclusion that the species 
described by these numerous investigators are simply phy- 
siological varieties of the same species. 
It is possible, in fact eminently probable, that changes of 
various sorts may have been brought about by differences in 
environment, and that all of the species which have been 
described may have come from similar iarval forms which 
have reached different hosts, and have there been subjected 
to influences which have been more or less conducive to their 
further development. 
In such a supposition I believe that I am warranted. The 
encysted Gasterostoma, differing from Bucephalus only in 
the absence of the tails, which I obtained from Menidia, as 
well as the free forms which I found in the intestine of the 
same host, had reached at the end of a week, after being fed 
