MR. T. H. HUXLEY ON THE AFFINITIES OF THE MEDUSAE. 
425 
Section II . — Of the Affinities of the Medusae. 
56. Certain general conclusions are deducible from the facts stated in the pre- 
ceding section. It would appear, — 
1st. That a Medusa consists essentially of two membranes inclosing a variously- 
shaped cavity, inasmuch as its various organs are so composed (7, 8, 14, 21, 22, 29, 
33, 38, 39, &c.). 
2ndly. That the generative organs are external, being variously developed processes 
of the two membranes (38, 39, 42, 48, 49) ; and 
3rdly. That the peculiar organs called thread-cells are universally present (7, 15, 
31, 32). 
Now in these particulars the Medusae present a striking resemblance to certain 
other families of Zoophytes. These are the Hydroid and Sertularian Polypes, the Phy- 
sophoridae and Diphydae, with all of which the same three propositions hold good*. 
57- But in order to demonstrate that a real affinity exists among different classes 
of animals, it is not sufficient merely to point out that certain similarities and analo- 
gies exist among them ; it must be shown that they are constructed upon the same 
anatomical type, that, in fact, their organs are homologous. 
Now the organs of two animals or families of animals are homologous when their 
structure is identical, or when the differences between them may be accounted for by 
the simple laws of growth. When the organs differ considerably, their homology 
may be determined in two ways, either — 1, by tracing back the course of develop- 
ment of the two until we arrive by similar stages at the same point ; or, 2, by inter- 
polating between the two a series of forms derived from other animals allied to both, 
the difference between each term of the series being such only as can be accounted 
for by the laws of growth. The latter method is that which has been generally em- 
ployed under the name of Comparative Anatomy, the former being hardly applicable 
to any but the lower classes of animals. Both methods may be made use of in in- 
vestigating the homologies of the Medusae -f-. 
58. A complete identity of structure connects the “foundation membranes” of 
* “ Les parois du tube nutritif sent formees d’une double membrane toujours rondee intimement dans cette 
partle du polype, I’externe repond aux teguments ; I’lnterne est xme continuation de la membrane digestive de 
la capacite alimentaire.” — Cuvier, Org. de Generation des Zoophytes, Lemons d’Anat. Comp. t. viii. 2nd edit. 
I have elsewhere pointed out that the same circumstance obtains among the Diphydse and Physophoridse. 
That the generative organs are external in the Sertularian and Hydroid Polypes has been long known. 
Milne-Edwards has shown that they have a similar position in one of the Physophoridae (^Apolemia) . I have 
observed it myself in the Diphydae. 
The presence of the thread-cells has been determined by Will in the Diphydae, by Milne-Edwards in Apo- 
lemia, by myself (only ??) in Physalia, Physophora, Athorybia and other Physophoridae, and in the Sertularian 
Polypes. 
f The above definitions may be thought needless and even trite, but the establishment of affinities among 
animals has been so often a mere exercise tke imagination, that I may be pardoned for pointing out the 
guiding principles which I have followed, and by which I would wish to be judged. 
