ON THE AFFINITIES OF THE FAMILY OF THE MEDUS/E 23 



sionally is thick and distinct from the inner, especially about the 

 circular marginal canal, for the walls of a distinct vessel. Even if this 

 be the case, what are the blood-corpuscles ? 



55. The thread-cells resemble in all respects those of the Diphydae, 

 which I have described elsewhere, consisting of a delicate outer cell 

 inclosing another thick-walled cell, with a spiral filament of greater 

 or less length, coiled up in its interior and capable of protrusion on 

 pressure. 



Section II.— (9/ the Affinities of the Medusa. 



56. Certain general conclusions are deducible from the facts stated 

 in the preceding section. It would appear, — 



1st. That a Medusa consists essentially of two membranes in- 

 closing 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 Medusje present a striking resem- 

 blance to certain other families of Zoophytes. These are the Hydroid 

 and Sertularian Polypes, the Phj'sophoridas 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 analogies 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 homo- 

 logous 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 deter- 



^ " Les parois du tube nutritif sont formees d'une double membrane toujours rondee 

 intimement dans cette partie du polype, I'externe xi^orA au.\ teguments ; I'interne est une 

 continuation de la membrane digestive de la capacite aliraentaire." — Cuvier, Org. de Genera- 

 tion des Zoophytes, Lejons d'Anat. Comp. t. viii. 2nd edit. 



I have elsewhere pointed out that the same circumstance obtains among the Diphydae and 

 Physophorid^. 



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 

 Physophoridje {Apolemia). 1 have observed it myself in the Diphydre. 



The presence of the thread-cells has been determined by Will in the Diphydee, by Milne- 

 Edwards in Azotemia, by myself (only ? ?) in Physalia, riiysophora, Athorybia, and other 

 Physophoridte, and in the Sertularian Polypes. 



