358 THE EVOLUTION OF MAN. 



Animals). In the ancestors of Vertebrates, the development 

 of the skeleton did not take place till much later, in the 

 Chorda Animals {Ckordonia). In them, after the sl^ia- 

 system and the intestinal system, two other organ-systems 

 simultaneously arise ; these are the nervous and the mus- 

 cular systems. The way in which these two organ-systems 

 which mutually condition each other, developed simulta- 

 neously and independently, in reciprocal action and yet in 

 opposition to each other, was first explained by Nicholaus 

 Kleinenberg in his excellent monograph on the Hydra, the 

 common fresh- water Polyp.^^'^ In this interesting little 

 animal, single cells of the skin-layer send fibre-shaped pro- 

 cesses inward, which acquire the power of contraction, the 

 capacity, characteristic of the muscles, of contracting in a 

 constant direction. The outer, roundish part of the exo- 

 derm ceU remains sensitive and acts as the nervous element, 

 the inner, fibre-shaped part of the same cell becomes con- 

 tractile, and, incited to contraction by the former part, acts 

 as the muscular element (Fig. 293). These remarkable 

 neuro-muscular cells thus still unite in a single individual 

 of the fii'st order the functions of two organ-systems. One 

 step further; the inner, muscular half of the neuro-muscular 

 cell (Fio-. 293, tyi) acquires its own nucleus, and separates 

 from the outer, nervous half {n), and both organ-systems 

 have their independent element of form. The fission of the 

 muscular skin-fibrous layer from the nervous skin-sensory 

 layer in embryonic Worms confirms this important ph3^1o- 

 genetic process (Figs. 50, 51, vol. i. p. 236). 



These four organ-systems, which have been mentioned, 

 were already in existence, when an apparatus developed, 

 tertiarily, in the human ancestral line, which, at first 



