348 Prof. AUman 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- 

 dusae 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, 

 ill the structure of the marginal tentacles, and even in 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 Hydroida 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 vaHd systematic characters, these 

 organs being especially liable to an increase in number as the 

 Medusa advances towards maturity. In some cases, however. 



