364 THE EVOLUTION OF MAN. 



connective tis.sne a secondary yjeriod, then we may cha- 

 racterize a third, much hater period, by nerve-muscle tissue. 



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Fig. 291. 



Fig. 293. — Nerve-muscle tissue. Three cells from Hydra : n, outer, 

 nervous ; vi, inner, muscular part of the cells. (After Kleinonberg.) 



Fig. 294. — Nerve-tissue (from a spinal nerve knot) : a, anterior, h, 

 posterior root of the spinal nerve ; d, e, fibrous nerve-stem ; /, g, h, i, nerve 

 cells in ganglion (/, unipolar, g, h, bipolar cells) ; 1-, I, nerve fibres. (After 

 Frey.) 



Fig. 295. — Muscle-tissue. Three pieces of striped muscle fibre (a). In- 

 tei-fibrons fat-cells (/.). (After Frey.) 



For while in the lowest Plant Animals the body consists 

 merely of covering tissue, and while in many other 



