130 THE EVOLUTION OF THE METAZOA 



seriously (though it is still occasionally met) which considers 

 the Turbellaria together with all Acoelomata and "Pseudo- 

 coelomata" (we call them Ameria) to be a result of a retrogres- 

 sive development ("degeneration"), and according to which 

 the simple nervous system of small species of Turbellaria must 

 be a secondary element. The specialists in Turbellaria all 

 disagree with such an interpretation. This interpretation has 

 been accepted only by those zoologists who would like to 

 derive, on the basis of the enterocoele theory, the perigastro- 

 coele from the intestinal cavities of anthopolyps, a plan which 

 cannot be condemned sharply enough. 



In view of the fact that in the Anthozoa the polyps certainly 

 have a more highly developed nervous system than the one 

 that can be found, in Hydra, we would be obliged, if we ad- 

 hered to the old interpretation of the development of Cnidaria, 

 to accept the proposition that they had progressively developed 

 their nervous system in spite of their unchanged sedentary 

 way of life. This contradicts all that is known about "the 

 influence of the sessile way of life." In a certain sense there 

 has really been a progressive development, as in the case of 

 the muscle system, yet only inasmuch it has led to a speciali- 

 zation which stands in a causal relationship to the sedentary 

 way of life. With regard to the whole structure, however, we 

 can observe in the nervous system of Cnidaria indications of 

 a progressive retrogression. It is only in the case of the pri- 

 marily soHtary and, even if slightly, moving Actiniae that a 

 clear condensation of nerve elements can be identified at their 

 aboral ends (the foot plate) (P. Groselj, and recently Pantin 

 and his school). A reduction of the nervous system can be 

 observed as early as in Anthozoa that have become colonial 

 animals. This reduction goes parallel to the reduction of the 

 muscle system and we can see, as the final result, the nerve 

 net limited to the oral region only. 



In Scyphopolyps and in Hydropolyps the condensation of 

 the nervous system disappears completely. In connection with 

 the secondarily soUtary Hydra which has again become an 



