CNIDARIA 73 



fold parts of the Siplionopliore body. They can consider 

 certain parts (polypites, dactylozoöids, etc.) as metamorphosed 

 polypoid individuals, other parts (nectocalyces, hydrophil I ia, 

 gonophores) as metamorphosed medusoid individuals, which 

 remain united with the colony. The adherents of the medusn 

 theory, on the other hand, have at their disposal only the 

 hydroid medusa as a fundamental form for the derivation of 

 all the numerous polymorphous parts of the Siplionopliore 

 organism, for only new medusas can ever be produced from 

 a medusa by budding. Since by this explanation the poly- 

 pites are homologized with the manubria, and the tentacles 

 with the marginal tentacles of a medusa, the adherents of 

 this theory find it necessary to assume an ancestral form in 

 which the medusa exhibited a bilaterally symmetrical 

 structure, while a single tentacle was advanced to the base 

 of the manubrium, and both these parts had emerged upon 

 the ex-umbrellar side of the medusa-bell through a fissure in 

 the umbrella— conditions which, as a matter of fact, do not 

 exist in any Hydromedusa. As a further consequence the 

 partisans of the medusa theory must assume the possibility 

 of a considerable dislocation of these difi^erent primary 

 organs and an extensive capacity of the individuals to 

 multiply different organs. With all these assumptions, 

 there arise certain difficulties which are not encountered in 

 the hydroid theory.' 



Even if the ancestral form of the Siphonophora assumed 

 by the medusa theory, and described above (which is re- 

 capitulated in ontogeny by the Siphonula-stage and by the 

 sterile person of the Eudoxige), were to be derived from 

 bilaterally symmetrical Anthomedusoe with only one marginal 

 tentacle (for example, from the Hybocodon belonging to 

 Corymorpha), it would still be difficult to point out in any 

 way the causes for the appearance of the fissure in the 

 umbrella and the described dislocation of the organs. The 

 difficulty is increased by the circumstance that these cha- 

 racters are lacking in the sexual individuals of the Siphono- 



1 It should be stated that recently Hatschek {Lehrbuch der Zoologie) 

 has introduced moditications into Haeckel's medusa theory, by means of 

 which a part of these ditHculties seem to be set aside. 



