Prof. Allman on the Hydroida. 



473 



I have here assumed, with Haeckel, the direct development of 

 Geryonia from the egg; so that Geryonia, n, non-sexual Medusa, 

 takes the place of a non-sexual polypoid trophosome. But, as 

 already said, there is no proof that Geryonia has not originated 

 as a bud from a polypoid trophosome ; and if so, G ought to be 

 preceded by T in the above diagram — an addition which would 

 merely increase the polymorphism without affecting the principle. 



In no case, however, is the polymorphism of the zooids greater 

 than what is well-known to occur among the Hydroida — as, for 

 instance, in Campanularia, where the polypoid trophosome gives 

 origin by budding to a gonoblastidium, and this to a gono- 

 blastocheme, which in its turn developes, by a similar non- 

 sexual act, the sexual bud or gonosac. The form of the dia- 

 gram of Geryonia, however, differs from that of Campanularia, 

 inasmuch as Campanularia presents a simple linear series, 

 while in Geryonia, in consequence of one and the same zooid 

 (the Geryonidan) producing two sets of heteromorphic buds 

 (the gonosac and the iEginidan), we have a series present- 

 ing two branches, which run off in different directions. 

 This, however, is exactly what occurs in Hydractinia echinata, 

 pi which the gonosac is borne not only on a gonoblastidium, 

 but also occasionally on the trophosome directly * . This 

 will be at once obvious if the diagram just given be com- 

 pared with the following, which represents the alternation in 

 Hydractinia, and in which the symbol GB is used to indicate 

 the gonoblastidium. It will then be apparent that the two have 

 precisely the same form. 



T + Gon|- 



+ 

 GB 



+ 

 Gon 



© 



GB 



+ 

 Gon 



T + Gon Q..&C 



GB 



+ 

 Gon 



© 



&c. 



&c. 



&c. 



&c. 



* As pointed out by Dr. T. Strethill Wright (Proc. Roy. Phys. Soc. of 

 Edinb. Nov. 1856). 



