CHAPTER XXII. 



THE MUSCULAR SYSTEM. 



IN all the Ccelenterata, except the Ctenophora, the contrac- 

 tile elements of the body wall consist of filiform processes of 

 ectodermal or entodermal epithelial cells (figs. 375 and 376 B). 

 The elements provided with these processes, which were first 

 discovered by Kleinenberg, are known as myo-epithelial 

 cells. Their contractile parts may either be striated (fig. 376) 

 or non-striated (fig. 375). In some 

 instances the epithelial part of the 

 cell may nearly abort, its nucleus 

 alone remaining (fig. 376 A) ; and 

 in this way a layer of muscles lying 

 completely below the surface may 

 be established. 



There is embryological evidence 

 of the derivation of the voluntary 

 muscular system of a large number of types from myo-epithelial 

 cells of this kind. The more important of these groups are the 

 Chaetopoda, the Gephyrea, the Chaetognatha, the Nematoda, and 

 the Vertebrata 1 . 



While there is clear evidence that the muscular system of a 

 large number of types is composed of cells which had their 

 origin in myo-epithelial cells, the mode of evolution of the 



1 If recent statements of Metschnikoff are to be trusted, the Echinodermata must 

 be added to these groups. The amoeboid cells stated in the first volume of this 

 treatise to form the muscles in this group, on the authority of Selenka, give rise, 

 according to Metschnikoff, only to the cutis, while the same naturalist states the 

 epithelial cells of the vasoperitoneal vesicles are provided with muscular tails. 



FIG. 375. MYO-EPITHELIAL 

 CELLS OF HYDRA. (From Gegen- 

 baur ; after Kleinenberg.) 



in. contractile fibres. 



