REPOET ON THE HYDROIDA. 
xlix 
PRIMARY MODIFICATIONS OF FORM.— CLASSIFICATION OF 
THE HYDROIDA. 
A complete system of the Hydroida would include not only all Hydroid trophosomes 
with their associated gonosomes, but all Hydromedusse, whether traced to polypoid 
trophosomes or not. There exist, however, many free Hydromedusae — both Antlio- 
medusse and Leptomedusae, — which have not yet been so traced ; but so closely do these 
correspond with Medusae, which are known to be budded off from fixed trophosomes, that 
there can scarcely be a doubt that, were we acquainted with their whole life-history, we 
should find that, like the Medusae which have been traced to hydriform stocks, these also 
are planoblasts which had originated as buds from fixed trophosomes. As, however, 
nothing is known of the trophosomes from which these Medusae have been derived, it 
has been generally deemed convenient to treat them independently as members of a 
general system of the Medusae, in which the Meclusal structure is made the basis of the 
classification, without necessary reference to the morphological details of the polypoid 
trophosomes . 1 
There is, however, another group of Craspedotae or Hydromedusae, into whose life 
series a polypoid term does not appear to have been ever intercalated, and which may 
accordingly be regarded as forming in themselves a separate and well-defined group of 
the Hydroida. 
It is proposed, therefore, in the following sketch of Hydroid classification, to include 
in the first place those Hydroids with whose trophosomes we are acquainted, making the 
characters of the trophosome the primary element in the classification ; and in the 
second place, such free Hydromedusae as there is good reason to believe are never derived 
from polypoid trophosomes. In the meantime, such free Anthomedusae and Lepto- 
medusae as. have not yet been traced to their trophosomes must be left to find their 
proper places among the former group as soon as the discovery of their trophosomes 
shall afford the necessary data. 
The order Hydroida, in the sense in which it is thus proposed to regard it, includes 
so many well-marked modifications of form, that the zoologist has no difficulty in finding 
among these such characters as may be legitimately used as the bases of natural 
systematic groups. 
A comparison of the members of the Hydroida with one another shows that — 
including the extinct Graptolites, whose allocation, however, among the Hydroida, 
depends on considerations of a more or less hypothetical nature — the order embraces 
within itself six primary types of form. 
1 See especially Haeckel, Das System der Medusen, Jena, 1879. 
(ZOOL. CHALL. EXP. PART LXX. — 1888.) 
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