238 THE EVOLUTION OF THE METAZOA 



interpretation of the origin and the evolution of Cnidaria, 

 and also of Hydrozoa, we are forced to change the sequence, 

 i.e. we consider Thecata as the more primitive group ot Hydro- 

 idea. A whole series of facts make it probable that the rich evo- 

 lution of Hvdroidea had begun with the Thecata. First we 

 must imagine the most probable way in which the Hydrozoa 

 eventually developed from Scyphozoa. It may prove useful to 

 trace even further back in phylogeny, back to the origin of 

 Scyphozoa. The reader will remember that I derive the Scy- 

 phozoa, especially their polyps, from the so-called "creeping" 

 and therefore stolonate cormi of Anthozoa (probably from 

 some forms that stood closest to the ancestors of Stolonifera, 

 among the recent Octactiniaria) that had developed a cuticular 

 periderm, (which can be easily distinguished from the skin 

 high over their anthocodium, the head of the polyp; Fig. 43). 

 It was from the Scyphozoa, which had the polyp form only 

 and which did not vet produce a medusa generation, that 

 the equally stolonate and creeping cormi of Hvdroidea was 

 in aU probabilitv developed, £rst in the form of hydropolyps. 

 The personal part of the hydropolyp had been further divided 

 into the basal and undifferentiated stem and into the differen- 

 tiated head (hvdranth). The hydrotheca with its special form 

 had evolved in several directions from the undifferentiated 

 peridermal part, reaching its dimas in those species which 

 have an automatically closing cover lid (Fig. 44). During this 

 phase of the progressive evolution, the Hydrozoa already be- 

 gan to produce their medusae, a development which was 

 independent from the otherwise similar morphogenetic 

 process of the formation of medusae that can be found 

 in Scyphozoa — as has already been repeatedly emphasized 

 in the present study; in Scyphozoa the medusae formed by 

 means of a transverse division, and in the thecate Hydroidea 

 from the separated buds of hydranths. This also agrees 

 with the fact that quite generally budding as a consequence 

 of a prolonged sessile way of life prevails over transverse 

 division. 



