116 THE EVOLUTION OF THE METAZOA 



fact that during the ontogeny, the Anlage of the nerve system 

 emerges in close connection with the Anlage of the ectoderm, 

 a fact which can especially be observed in the higher developed 

 animals. 



The destiny of the muscular system of Cnidaria has been 

 completely determined by the transition to the sessile way of life 

 and that in two directions. Due to the formation of colonies 

 (cormogony) which they had early begun to develop, the polyps 

 as dependent "persons" or subindividuals have grown smaller 

 and smaller (a progressive diminution). It is because of this fact 

 that the smallest subindividuals can be found, as this can be 

 expected, in the Hydrozoa. It is evident that the regressive 

 development of the whole muscular system has been condi- 

 tioned by this fact. On the other hand, and because of the 

 emerging restriction and the change of the role of the muscular 

 system, a cutaneous muscle tube has been formed which is 

 t3rpical of Cnidaria ; a layer of longitudinal muscular fibres under 

 the skin, and another layer of circularly placed muscular fibres 

 under the intestinal epithelium. We have here actually two 

 types of movement only: retraction which is followed by ex- 

 tension, and nutation (curving), due to the contraction of 

 individual sectors of the layer of longitudinal fibres. In species 

 whose solitary life is a secondary phenomenon, e.g. in Hydra, 

 this has led to a new type of movement, such as can be ob- 

 served in the loop -worm or in Hirudinea. 



The anthopolyps have inherited a richly developed muscular 

 system from their turbellarian ancestors. Besides a strongly 

 developed cutaneous muscle tube which consists of three 

 body layers there are also "internal" ("gastrodermal") muscular 

 fibres that show a particularly strong development in the sarco- 

 septa and as the sphincter of the oral area, and finally the dor- 

 soventral and the obliquely placed lateral muscle fibres. In the 

 primitive, solitary anthopolyps which stand erect and gUde 

 slowly over the sea bottom, it was the middle layer with its 

 diagonally placed muscular fibres which was first lost because 

 it had become superfluous. Thus two external layers only have 



