288 
We find f. inst. that the hydrothecae of Lafoea fruticosa aud L. 
gracillima show a distinet adcauline convexity. As to the other 
sertularian characters the genera Perisiphonia, Cryptolarici and Gram- 
maria have their hydrothecae adnate to the hydrocaulus in a larger 
or smaller part of their adcauline wall, and in the two last named 
genera this connection is as in the Sertulariidae inseparable, while 
in Perisiphonia it cun be loosened by the aid of reagents. As, 
therefore, the above named forms cannot by a single character be 
divided from the Lafoeidae , I, on the contrary, find it arbitrary 
to refer them to the Sertulariidae, and at the same time there 
cån be no doubt, but that the latter family gains in firmness 
and coherence by only embracing operculate forms. 
In animals of so simple a structure as the Hydroid polyps we 
can only expect to find a few distinguishing marks between the 
systematic divisions, and most families and genera are only divided 
from each other by one or two characters. The two chief divisions, 
the Athecata and the Thecaphora are only divided from each other 
by the presence in the latter of more or less developed protective 
cases for the hydrants and the gonophores, and it seems reasonable 
to ascribe systematic significance also to the operculum, a structure, 
which must be regarded as the complement of the protective cases 
and, so to speak, as the end-result of the same effort, which has led 
to the formation of the hydrothecae and gonothecae. We may 
farther point out as an evidence of its systematic importance, that 
it has that in common with other structures of svstematic signifi¬ 
cance tliat it presents a rich development of characteristic modifi- 
cations which give excellent generic characters. 
We may now regard the relation between the Lafoeidae and 
the Gampanulinidae. If we compare the two above family diag¬ 
noses we shall find that the two families are only sharply divided 
by a single character, namely the presence or absence of an oper¬ 
culum, all the other characters being more or less relative, and, 
therefore, there can be no doubt, tliat they are very nearly related 
