443 . Dr. E. Haeckel on the Relationship of 



heterogonism or allceo genesis. It is not, as in the other multi- 

 farious forms of the alternation of generations, a sexual and an 

 asexual form — a Medusa and a polype — that stand in a recipro- 

 cal genital relation to each other ; but we find here that a per- 

 fectly developed Medusa, evolved by metamorphosis from a 

 larval form, at a time when its generative organs furnish mature 

 products (from which probably the above-mentioned larvae are 

 produced), produces young Medusae, asexually, by formation of 

 buds in its stomachal cavity, and these are developed into a 

 Medusa-form perfectly distinct from their parent, and which in 

 its turn becomes sexually mature. 



What, then, becomes of the sexual products of Cunina ? How 

 does this octoradiate ^ginide revert to the sexradiate Geryonide? 

 Or does it only propagate in an ^ginide form ? or are the larvae 

 of the Geryonia produced sexually or asexually from the Cunina? 

 But, also, what becomes of the sexual products of the Geryonia ? 

 Does the Cunina also propagate asexually ? or are there Hydroid 

 polypes which establish the union between the two Medusoid 

 forms, which appear to be so widely separated? These and 

 many other questions force themselves upon us in the presence 

 of this wonderful fact, without our seeing at present any way of 

 escaping from this labyrinth. But I hope shortly to be able to 

 take these questions in hand again at the Mediterranean, and to 

 bring them to a solution. 



The paradoxical nature of the demonstrated relation might 

 well lead us to a suspicion of parasitism. But, leaving out of 

 consideration other pertinent reasons to the contrary, this is at 

 once contradicted with certainty by the fact that the develop- 

 ment of the Cunina-huds upon the surface of the tongue of the 

 Geryonia may be traced through all stages from its first com- 

 mencement. The first foundation of the sprouting bud is no- 

 thing but a small disciform thickening of the epitheliuui of the 

 tongue. This homogeneous cell-growth is then differentiated 

 into two distinct laminae — a lighter ectoderm and a darker en- 

 doderm. In the latter is produced a small round cavity, the ' 

 first trace of the stomachal cavity, which then grows out into 

 the above-mentioned cyUndrical stomachal tube, whilst the disk 

 is differentiated into eight segments. 



The JEginida and GeryonidcR have hitherto passed as perfectly 

 distinct families of Medusae. The numerous peculiarities which 

 so strongly characterize both the external form of the body and 

 the interior structure of the j^ginidce appear, indeed, to remove 

 this family far from all others. Quite recently, in fact, two 

 distinguished naturalists have even separated the ^ginidm alto- 

 gether from the great section of the Craspedota [Cryptocarpa) . 

 Fritz Miiller has placed them as a third distinct primary group 



