TERATOLOGY AND PATHOLOGY. 203 



The only explanation which it seems possible to suggest is, that in the twin medusae 

 we have a case of accidental adhesion contracted between two neighbouring buds while still con- 

 nected with the trophosome, this adhesion having been followed by a free communication between 

 the two umbrella cavities. I never met with more than a single example, and, whatever ex- 

 planation we may be disposed to offer as to its origin, it seems evident that it cannot be regarded 

 as otherwise than abnormal. 



The other case to which I would here wish to draw attention consists in a change of form 

 observed also in medusas of Syncorijne ^ndchella, whicli had been thrown off from trophosomes con- 

 fined in my jars. Li none of these medustp, though they had remained under observation for 

 nearly a month, had any development of generative elements taken place, but they had all under- 

 gone a very remarkable change. 



The commencement of this change might have been noticed a few days after their liberation. 

 The umbrella became everted, and then began to diminish in size, contracting from its margin 

 towards its summit, until in a few days it had almost entirely disappeared, being then merely 

 represented by a thick disc of a somewhat quadrangular form, which projected round the base of 

 the manubrium. Each of the four angles of this disc was continued into one of the marginal 

 tentacles, whose proximal end, following the contraction of the umljrclla, had been thus brought 

 upon a level with the base of the manubrium. The interior of the disc was occupied by a cavity 

 which communicated freely with that of the manubrium, and with that of each of the four tentacles 

 which extended from its angles. With the contraction of the umbrella the circular canal and 

 velum had disappeared, and the radiating canals were now represented solely by the short 

 channels by which the interior of the hollow disc communicated through the thickness of its walls 

 with the tubes of the tentacles. Neither tentacles nor manubrium had undergone any material 

 change ; the former retained their full power of extension and retraction, and the latter all its 

 original irritability — moving from side to side, lengthening and shortening itself, opening and 

 closing its mouth with at least as much vigour as before the disappearance of the umbrella. The 

 medusa in this condition reminded us strongly of the gonophore of Clmatella, though the degrada- 

 tion of the umbrella was more complete than in the latter. The medusa had, in fact, become 

 changed by a retrograde metamorphosis into a hydranth. 



Changes had been noticed also by Dujardin in the medusa of his Syncoryne deci- 

 piens ; but he had not followed them beyond an eversion of the umbrella, which is pro- 

 bably the commencement of the changes residting in the disappearance of this part of the 

 structure. 



Notwithstanding the very striking character of the changes now described, and their 

 resemblance to a normal metamorphosis, I cannot see in them anything more than a degradation 

 of structure resulting from imperfect nutrition — a mere forerunner of complete disintegration 

 and death. They are, however, most instructive in their bearing upon the homologies between 

 the medusa and the hydranth, and completely support the view that the radiating canals of the 

 medusae are the homologies of the channels by whicli the gastric cavity of the hydranth is con- 

 tinued through the thickness of its walls into the interior of the tentacles, which will then repre- 

 sent those marginal tentacles of the medusa which constitute the continuations of its radiating 

 canals. 



Colonies of Ilydradinia echinata may occasionally be met with in which certain hydranths 

 have become bifurcated at some distance from the base, each branch of the bifui-cation carrying 



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