Limitation of Genera among the Hydroida. 349 
we may fairly assume the number presented by the marginal 
tentacles in the young Medusa as representing their permanent 
condition, as, for example, in the single long tentacle of the 
Medusa in Corymorpha, where we find, by going back to the 
early stages of the development of this Medusa, that the pecu- 
liar asymmetrical form which, in a later stage, finds its expres- 
sion so decidedly in the great development of a single tentacle 
is quite apparent before any trace of a tentacle can be detected. 
With regard to nomenclature, I am convinced that, except in 
certain special cases, we must give to our Hydroid the name 
under which it was first described, whether this name may have 
been originally given to the trophosome or to the gonosome. 
The fact of our giving as a generic name to the complete Hy- 
droid that by which the Medusa had been previously known 
needs not prevent our employing the same name for all those 
similar Medusz whose trophosome has not yet been discovered ; 
but we must keep in mind that the name, when used in this 
sense, is purely provisional, and liable to be changed when the 
discovery of the trophosome shall determine the true genus of 
our then no longer incomplete Hydroid. 
It is upon the principles here urged that I have drawn up the 
following synopsis of the genera and species of the Tubularian 
and Campanularian Hydroids.. I have confined myself, how- 
ever, entirely to those forms in which the trophosome is known, 
the numerous free Meduse which have not been traced to a 
trophosome, or been proved to originate by direct development 
from the egg, holding places in our system which must for the 
present be regarded as altogether provisional. 
In the generic descriptions I have adopted as far as possible a 
uniformity in the selection of characters and in the order in 
which these characters are noted; and | have further, by availing 
myself of terms already in use, and by introducing one or two 
new ones, been able to avoid tedious circumlocution, and to 
condense the descriptions without sacrificing their precision. 
In order that the synopsis may be more easily followed, it 
will be well to give here definitions of the principal terms used, 
while for a fuller exposition of the terminology of the Hydroida 
I must refer to Prof. Huxley’s ‘Oceanic Hydrozoa,’ published 
by the Ray Society, and to some papers of my own, more espe- 
cially a paper “On the Structure and Terminology of the 
Reproductive System in the Corynide and Sertulariade,” pub- 
lished in the ‘Annals and Magazine of Natural History’ for 
July 1860, and a “ Report on the Reproductive System of the 
Hydroida,’” in the Report of the Newcastle Meeting of the 
British Association, 1863. 
The terms “ trophosome” and “ gonosome” have been already 
