35§ 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 (Chordonia). In them, after the skin- 

 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. 190 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 cell 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 first order the functions of two organ-systems. One 

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

 cell (Fig. 293, m) 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 phylo- 

 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 



