16 THE EVOLUTION OF THE METAZOA 



system has been developed in the same place. This organ has 

 been inherited by the Ctenophora from their turbellarian ances- 

 tors, and during the transition it came completely to the surface. 

 This agrees perfectly with the orientation as well as with the 

 way of life of Ctenophora. We must not forget in this con- 

 nection that the Ctenophora must in all probability be con- 

 sidered to have evolved by means of neoteny from the organi- 

 zationally simpler planktonic larvae. Naturally, evolution has 

 proceeded until the present forms of Ctenophora have been 

 reached. The oral opening was transplaced from the vicinity of 

 the aboral pole (first the anterior pole, and in Ctenophora the 

 upper pole) to the vegetative pole (in the creeping Turbellaria 

 the posterior pole, and in Ctenophora which are poised in water 

 the lower pole) (see Fig. 2). 



Medusae, too, regardless of whether they are Hydromedusae 

 or Scyphomedusae, live as planktonic animals poised in water, 

 not with their "head turned upwards" as in the case with 

 Ctenophora, but rather with their "head turned downwards," 

 i.e. with their vegetative pole turned upwards. Thus the oral 

 opening of medusae is turned downwards; its place corres- 

 ponds to that of the animal pole since they have developed 

 from polyps either by means of gemmation budding, (Hydro- 

 medusae), or by means of transverse fission (Scyphomedusae), 

 or secondarily, directly from ova. The polyp, the initial form 

 of Cnidaria, "stands" upright and is attached with its vegetative 

 or posterior pole to the floor of the sea. The medusa, which 

 has evolved from the oral part of the polyp, underwent a re- 

 version when it became a freely-swimming animal and turned 

 its originally anterior part upwards. Yet this uppermost part 

 of the medusa is no longer a part of its head as it has remained 

 in the ancestors of polyps, but instead the oral part has been 

 removed and is now, so to speak, only at the end of a stem. We 

 find therefore the surface of the so-called exumbrella turned 

 upwards when the animal rests, and forward when the animal 

 moves. The exumbrella is genetically and morphogenetically 

 empty because it is covered with one layer only of the ectoderm 



