312 THE EVOLUTION OF THE METAZOA 



be observed in numerous Metaphyta and in Spongiae. Yet 

 at the same time it can also be stated that great progress 

 has never been made along this way towards a definite whole. 

 The circumstances led to a change of exactly those metabionts 

 that had evolved by way of colonial formations to the sessile 

 way of life. This has proved an obstacle to a subsequent 

 progressive development. A "blastuloid" organization (Vo/- 

 vox) represents the highest level reached in the evolution of 

 the freely moving Metaphyta. Bypassing the immobile Meta- 

 phyta in which no central organs have been developed we see 

 that the sponges have not come very far in their evolution 

 either. They again have not developed any central organs or 

 any devices which would unite the whole aggregation into 

 a higher organic entity; they represent a very lowly-developed 

 side branch of the animal w^orld. 



In the numerous, mainly sessile Eumetazoa which had fre- 

 quently adopted a freely moving way of life (Siphonophora, 

 Ectoprocta, Taliacea) and w^hich had developed their own 

 cormi ("colonies") higher units can be developed from several 

 originally independent individuals. These new units represent 

 a new organization, they have developed their own indi- 

 viduality, and it is not even easy to identify their original 

 complexity. Siphonophora may serve here as the best and 

 probably also as phylogenetically the oldest example. They 

 have certainly reached the highest level of development in 

 this direction. They excel in a highly developed external 

 homogeneity, they show a considerable polymorphism both 

 in their polypoid and medusoid "persons" (the formerly 

 independent individuals), they have even evolved new^ im- 

 personal and thus specifically cormic organs, e.g. a pneuma- 

 tophore. Yet on the other hand they have not develop- 

 ed any centralized nervous system, neither have they been 

 able to overcome the limitations of Hydrozoa and to de- 

 velop in this way into a new animal type; this in spite of the 

 fact that they have become at least slightly freely moving 

 animals. 



