406 THE EVOLUTION OF THE METAZOA 



Specific character: here the anterior part of the animal evolved 

 into a scolex by means of which the animal becomes attached 

 to the intestinal wall of its host while the remaining part 

 of its body swims freely in the fluid and nutritive contents of 

 the intestine. It is therefore not surprising to find in these 

 animals an oppulent growth which is followed by a poly- 

 merization that can be observed above all in their complex 

 hermaphroditic genital apparatus, especially since we can also 

 quite frequently find a polymerization in the ancestors of 

 the Cestoda, in the Turbellaria. The development of proglot- 

 tides as a kind of a superstructure, a regulated segmentation 

 of the body where the individuality is preserved in spite of 

 a secondary separation of individual segments or of groups 

 of segments, are all purely results of a further adjustment to 

 such a specialized way of life. Yet even in this way no poly- 

 merous animal had ever been able to evolve. The state of 

 "stfobila" had evolved in an entirely different way and under 

 completely different circumstances than did the genuine poly- 

 mery (segmentation). The Cestoda are a blindly ending side 

 branch of the Ameria which could never have been able 

 to evolve into a higher form; they can therefore not be used 

 as a pattern by means of which we could explain the evolution 

 of a new higher type of segmented animals. They represent 

 a very interesting special case only of polymerization; 

 yet it must be mentioned in this connection that not every 

 polymerization led to a segmented state. 



As regards the "indications" that can apparently be observed 

 in the ontogenies we can clearly see if we view this problem 

 from our standpoint that they cannot be used as proof in 

 favour of the zooidal theory. Ontogeny has been consider- 

 ably changed since the days when the primitive and regulated 

 segmentation first evolved from the primitive Ameria with 

 a scattered polymerization. This change is characterized by the 

 emergence of a larval stage which remains a longer time in 

 plankton. We cannot therefore expect to find recapitulations 

 here any longer. Especially great wxre the changes which 



