468 Prof. Allmau on the Hydroida. 



channels by which the interior of the hollow disk 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 Clavatella, though the 

 degradation of the umbrella was more complete than in the latter. 

 The Medusa had in fact become changed by a retrograde meta- 

 morphosis into a polypite. 



Changes had been noticed also by Dujardin in the Medusa of 

 his Syncoryrie decipiens ; but he had not followed them beyond 

 an eversion of the umbrella, which is probably the commence- 

 ment of the changes resulting 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, 



1 cannot see in them anything more than a degradation of struc- 

 ture 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 polypite, and completely support the view that 

 the radiating canals of the Medusa? are the homologues of the 

 channels by which the gastric cavity of the polypite is continued 

 through the thickness of its walls into the interior of the tenta- 

 cles, which will then represent those marginal tentacles of the 

 Medusa which constitute the continuations of its radiating 

 canals. 



II. The Production of an JEginidan by gemmation from 

 Geryonia. 



A discovery of great importance in its bearing on the true 

 relations of the jEginidce has been just announced by Ernst 

 Haeckel*, who has seen a Medusa belonging to the family of 

 the Geryonidce {Geryonia hastata, Haeck.) giving origin, within 

 the cavity of its manubrium, to buds which, instead of repeating 

 the form of the Geryonia, become developed into a species of 

 Cunina {Cunina rhododactyla, Haeck.), a Medusa belonging to 

 the aberrant and hitherto perplexing group of the JEginida. 



Further, according to the author's views, the bud-producing 



* " Ueber erne neue Form des Generationswechsels bei den Medusen, 

 und liber die Verwandtschaft der Geryoniden und iEginiden." Auszug 

 aus dera Monatsbericht der Konigl. Akad. der Wissensch. zu Berlin, 



2 Feb. 1865. [Translated in the present Number of this Journal.] 



