214 THE EVOLUTION OF THE METAZOA 



have usually (though by no means always) animal groups which 

 developed directly from some related animals that lead 

 a sessile way of life and which as such reproduce asexually. 

 As examples we can mention, among Cnidaria, some species 

 of hydromedusae (above all those that belong to the Hydro- 

 idea); hydropolyps that have become secondarily solitary 

 animals; the Thaliacea among Tunicata; two secondarily soli- 

 tary species of Ectoprocta ( Monobryof^oon amhulans and M. 

 limcola), and perhaps t]\tL.oxosoma species among Endoprocta 

 (it is possible that these are primarily solitary animals !). 



In agreement with the rule that a tendency to asexual repro- 

 duction can be observed in sessile animals the Cnidaria (in 

 clear contrast to Ctenophora!) show asexual reproduction in 

 widely adopted and manifold ways. This has led in all three 

 subclasses to the formation of cormi — these are least developed 

 in Scyphozoa— and in this way to polymorphism (polyp- 

 medusa, polypean polymorphism, medusan polymorphism). It 

 is usually supposed, even if it is not always explicitly stated, 

 that in Cnidaria this type of reproduction appeared, so to speak, 

 spontaneously as a consequence of a general plasticity. This 

 has been linked with the regenerative ability. This, however, 

 is an explanation born of a dilemma. Reproduction by means 

 of transverse division can be found in Turbellaria, even if it 

 is not of quite a general distribution and it occurs in spite of 

 the fact that they live as freely-moving animals. Here, too, 

 attempts have been made to bring this property into a causal 

 connection with the well-developed ability to regenerate. 



These conditions become clear and understandable if viex^ed 

 from the point of view of my concept (Cnidaria as descendants 

 from Turbellaria). First it should be mentioned that there 

 are— besides specialized forms of asexual reproduction, lacera- 

 tion, formation of podocysts, cladogony, etc.— mainly two 

 types of asexual reproduction: division and budding. Of these 

 two types, division seems to be the more primitive form, because 

 it appears early in the freely living Protozoa, especially in 

 Flagellata and in Ciliata ; in Flagellata as a longitudinal division. 



