Limitation of Genera among the Hydroida. 347 
History Review’ (1861-63), where they appear under the form 
of a review of the work of Agassiz just referred to—papers in 
which there is no difficulty in recognizing the pen of an accom- 
plished zoologist who holds a chair in an Irish University, and 
is already well known by his valuable contributions to the lite- 
rature of the Celenterata. 
I believe that henceforth no classification of the Hyprorpa 
will be admitted by the zoologist which does not include in the 
conception of every Hydroid both those parts which are destined 
for the nutrition of the colony and those which are destined for 
the sexual perpetuation of the species, whether these last are in 
the form of fixed sacs or of free locomotive Medusz. 
It must be borne in mind that every Hydroid whose life- 
history has come fully before us consists (with only a single 
positively proved exception*) of two sets of zooids. One of 
these is destined for the nutrition of the colony, and has nothing 
to do with true generation; while the other is, on the con- 
trary, destined for true generation, and has nothing to do with 
the nutrition of the colony. For the whole assemblage of the 
former I have elsewhere+ proposed the term “trophosome,” and 
for that of the latter the term “ gonosome ;” and whether the 
gonosome remains permanently attached to the trophosome or 
becomes in whole or in part free, attaining thereby an indepen- 
dent existence, it is equally necessary that it should take its 
place in our diagnosis of genera and species. An adequate con- 
ception of the Hydroid can thus only be obtained by regarding 
it as the product of two factors, one of them finding its expres- 
sion in the trophosome, and the other in the gonosome. 
Now the characters to which we shall be justified in assigning 
a generic yalue will be found in both of these factors. The 
trophosome will present them chiefly in the form of the poly- 
pite, including the arrangement and structure of the tentacles 
(whether these be scattered or in one or more verticils, or whether 
they be filiform or capitate), in the solitary or associated condi- 
tion of the polypites, and in the nature and extent of the chiti- 
nous periderm. In the gonosome, characters of generic value 
will be found in the mode of origin of the gonophores and in 
their general form—whether they be in the condition of a fixed 
sac (adelocodonic) or of a developed Medusa (phanerocodonice) ; 
while each of these forms of gonophore may itself present dif- 
ferences which will afford characters of value in the limitation 
of our genera. It is true that among the adelocodonic forms it 
it is rare to meet with any differences so well marked as to be- 
* See my “Report on the Reproductive System of the Hydroida,” in 
the Report of the Newcastle Meeting of the British Association, 1863. 
~ + Loe, cit. 
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