Prof. Allman on the Hydroida. 469 



Geryonia is in a state of sexual maturity, and the buds which 

 are produced by it become developed into sexual Cunince. And, 

 still further, from having observed in the sea various free-swim- 

 ming forms which he regards as different stages in the develop- 

 ment of the Geryonia, he considers it probable that this Medusa 

 is produced by a process of direct development from the ovum, 

 his observations in this respect coinciding with those of Fritz 

 Miiller on the nearly allied Liriope catharinensis*. 



Of the phenomena thus observed he gives the following ge- 

 neralized statement : — A perfectly developed Medusa which has 

 been produced by metamorphosis from a larva, and is capable of 

 sexual multiplication, gives origin, by a process of budding in 

 its stomach-cavity, to young Medusae, which develope themselves 

 into perfect sexual forms entirely different from that of the Me- 

 dusa from which they spring. And in these facts the author 

 believes that there is presented an entirely new type of alterna- 

 tion of generations — a type totally different in its fundamental 

 principles from the phenomena which have been hitherto in- 

 cluded under this name, and one for which a new term is 

 needed. 



While the observations of Haeckel, however, can scarcely be 

 too highly estimated for the light they throw upon the relations 

 between the Geryonidce and JEginidce, it appears to me that he 

 greatly overrates the difference between the genetic phenomena 

 which are here presented and those already well known among 

 the Hydroida. 



It must be kept in mind that the essence of alternation of 

 generations consists (1) in the interposition, between every two 

 acts of true generation, of one or more acts of non-sexual multi- 

 plication; (2) in the fact that the heteromorphic elements in 

 an alternation are invariably connected with one another by a 

 non-sexual and not by a sexual genesis; (3) in the fact that 

 these elements exactly repeat themselves after each generative 

 act. 



Now in the present case, admitting that Haeckel has given a 

 correct interpretation of the phenomena, we have the develop- 

 ment of an ovum resulting by direct metamorphosis in a Geryo- 

 nidan Medusa which produces by non-sexual reproduction an 

 JEginidan Medusa, this last being sexually perfect, so that it 

 gives origin to fertile ova. So far the phenomena would come 

 exactly within the ordinary laws of alternation of generations ; 

 but a disturbing element is introduced by the fact of the Geryo- 

 nia not only giving origin to buds, but also producing fertile 

 ova. This is certainly an anomaly. I know of no other instance 



* Wiegmann's Archiv, 1859, p. 310. 

 Ann. % Mag. N. Hist. Ser. 3. Vol. xv. 32 



