THE EVOLUTION OF MAN. 



364 



connective tissue a secondary period, then we may cha- 

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



FIG. 94. 



FIG. 293. Nerve-muBcle tissue. Three colls from Hydra : n, outer, 

 nervous ; m, inner, muscular part of the cells. (After Kleinenberg.) 



Pio. 294. Nerve-tissue (from a spinal nerve knot) : a, anterior, b, 

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

 cells in ganglion (/, unipolar, g, h, bipolar cells) j fc, I, nerve fibres. (After 

 Frey.) 



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

 terfibrous fat-cells (b). (After Frey.) 



For while in the lowest Plant Animals the body consists 

 merely of covering tissue, and while in many other 



