Prof. Allman on the Hydroida. 467 



from a group of Syncoryne pulchella in one of my jars, I was 

 struck by observing two of these Medusse united to one another 

 by a small space on the convex surfage of their umbrellas, at a 

 short distance from the summit. One of the united Medusse 

 was a little smaller than the other ; but otherwise they were both 

 equally developed, and presented the ordinary condition of these 

 zooids at the time of their liberation from the trophosome. The 

 cavities of the two umbrellse freely communicated with one an- 

 other through the surface of junction. 



That neither of the Medusse thus so intimately united had 

 been produced by a bud from the other was evident ; for the 

 original point of union with the trophosome and the canal by 

 which the cavity of the manubrium had at one time communi- 

 cated with the somatic cavity of the trophosome were still dis- 

 tinct in each; while these facts are also opposed to the view 

 which would regard the twin Medusse as representing a single 

 one in the process of self-division. 



The only explanation which it seems possible to suggest is that in 

 the twin Medusse we have a case of accidental adhesion contracted 

 between two neighbouring buds while still connected with the 

 trophosome, though it is difficult to see why this adhesion should 

 have been followed by a free communication between the two 

 umbrella-cavities. I never met with more than a single example; 

 and, whatever explanation we may be disposed to offer as to its 

 origin, it seems evident that it cannot be regarded as otherwise 

 than an abnormal occurrence. 



Though many of the Medusse which, nearly a month ago, 

 had become liberated from the trophosome are still living in 

 my jars, no formation of generative elements has taken place in 

 any of them. They have, however, all undergone a very remark- 

 able change. 



The commencement of this change might have been observed 

 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 disk of a 

 somewhat quadrangular form, which projected round the base of 

 the manubrium. Each of the four angles of this disk was con- 

 tinued into one of the marginal tentacles, whose base, follow- 

 ing the contraction of the umbrella, had been thus brought 

 upon a level with the base of the manubrium. The interior of 

 the disk 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 



