52 THE EVOLUTION OF THE METAZOA 



the primary, or the main form out of which the polyps can 

 be considered to have evolved. On the other hand, the solitary 

 forms of the Hydrozoa polyps, and of polyps that have no ge- 

 neration of medusae have also been believed to be the pri- 

 mitive forms. Both these interpretations are equally wrong; 

 both of them are based on pre-suppositions that are methodo- 

 logically wrong. 



One more point can be of good service in our phylogene- 

 tic studies. This is the relationship betu^een locomotion 

 on one hand and the environment and the kind of feeding on 

 the other. As is well known, three stages in the evolution of 

 the active forms of locomotion must be taken into considera- 

 tion (cf. the study by L. Zenkewitsch). We disregard two 

 forms that occur in Protozoa: the amoeboid movement and 

 the movement effected by the individual flagella (it is true that 

 in Spongiae the amoeboid type of movement does occur, 

 even if to a small degree only; cf. Hadzi, 1917). In the small 

 and primitive groups of Ameria the type of movement corre- 

 sponds mainly to conditions that can be found among Protozoa, 

 in the Infusoria; this is movement by means of cilia and this 

 type of movement is preserved in planktonic larvae, so to say, 

 right through one classificatory system. The ciliary pulsation 

 produced by animals while they are exposed to the rhythmical 

 swell of waves enables them not only to glide over a firm 

 ground, which is true for the majority of Turbellaria, but also 

 to swim in free water, as can be observed in Infusoria and 

 Rotatoria. The formation of undulipodia (flagella and cilia) by 

 individual cells belongs to a general inventory of all animals. 

 There have been a few specialized animals however, e.g. 

 Nematoda among Ameria and insects among Polymeria, which 

 have completely dispensed with the action of undulipodia be- 

 cause the surface of these animals had been strongly cuticu- 

 larized. 



Undulipodia which cover the free upper surface do not serve 

 the purpose of locomotion only; they can also be transplaced, 

 together with their maternal cells, into the interior of the 



