348 Prof. Allman on the Construction and 
come of generic value; but among the phanerocodonic forms 
the differences are numerous and important—differences which, 
though they are fully recognized so long as we regard the Me- 
dusze as independent organisms, are yet usually ignored when we 
see in the Medusa only the sexual bud of a polypoid trophosome. 
They will be found in the form of the umbrella, in the form and 
development of the manubrium, in the situation of the generative 
elements, in the number and distribution of the radiating canals, 
in the structure of the marginal tentacles, and even im their 
number when we have reason to regard this number as perma- 
nent and not merely as the result of an immature condition, 
and, finally, in the presence or absence of lithocysts, and even in 
the position which these bodies hold on the umbrella-margin,— 
all which characters, either singly or combined, will afford valid 
grounds for the construction of our generic groups. 
The classification of the Hyprorpa would be a comparatively 
simple task if, as has been erroneously asserted, generically 
identical medusoids always arose from generically identical poly- 
poids, and, on the other hand, that generically identical polypoids 
always gave origin to generically identical medusoids. 
This, however, is far from being the case; and the history of 
the Hydroida renders us acquainted with two phenomena which 
signally break the uniformity assumed in the above propositions. 
The phenomena to which I refer are, (1) the association of 
similar gonosomes with dissimilar trophosomes (isogonism) ; and 
(2) the association of dissimilar gonosomes with similar tropho- 
somes (heterogonism). The difficulties which these phenomena 
throw in the way of a natural classification of the Hydroida 
may be compared to those which the mineralogist meets with 
when he finds isomorphism and dimorphism interfering with the 
uniformity of his mineralogical system. 
But the great difficulty, after all, in the application of the 
method here advocated is found in the fact that the Medusa at 
the time of its liberation is still in an immature state, and may 
be destined to undergo important changes before arriving at its 
adult condition. In such cases, unless we have succeeded in 
following the Medusa to its ultimate form, our determination of 
its type must be regarded as only approximative. Analogy, 
however, will greatly aid us in this determination, by pointing 
out what are the parts most liable to change, and what the 
direction in which this change is likely to take place. 
From these considerations we learn that the number of mar- 
ginal tentacles in the recently liberated Medusa must be accepted 
with great caution as affording valid systematic characters, these 
organs being especially liable to an increase in number as the 
Medusa advances towards maturity, In some cases, however, 
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