170 



Elementary Zoology. 



the same great division as that which contains the hydra, we 

 get a further evolution of the muscle-cell. In such cases (see 

 Fig. 77) the fibre is much more important than the cell of 

 which it is a formation. Nevertheless, the cellular constitution 

 of a muscle-fibre is plainly evident. 



In the higher animals, as has already been pointed out in 

 the case of the earthworm, the cellular Constitution of the 

 muscle-fibres is not always evident. It is, however, during 

 the course of the development of the fibres. Fig. 78, for 

 example, which represents a muscle-fibre of an embryo sheep, 

 is absolutely like the muscle-fibre of an adult medusa. The 



Fig, 77.— Muscle-cells of Cselenterate. 

 (From Claus-Sedgwiclc's " Zoology."") 



Fig. 78. — Developing muscular fibres 

 of fo2tal Sheep. Highly magnified. 

 CAfter Wilson Fox, from Quain*s 

 " Anatomy.") 



cell here is partly metamorphosed into a fibre. A bundle of 

 the fibres closely associated together with a more complete 

 disappearance of the original protoplasm of the cell is a 

 muscle-bundle. In the case of the medusa, and in others, 

 each cell gives rise to a single fibre only; but in the earth- 

 worm, as already mentioned, several fibres appear in the interior 

 of each cell. But this is not a difference of importance ; neither 

 is the distinction between the striate fibres of all vertebrates 

 and some invertebrates and the smooth muscular fibres of 

 vertebrates and invertebrates a matter of fundamental dis- 

 tinction. The striated fibre consists of muscular substance, 



