CHAPTER XXII. 



THE MUSCULAR SYSTEM. 



FIG. 375. MYO-EIITHELIAL 



CELLS or HYDRA. (From Gegen- 

 baur ; after Kleinenberg.) 



m. contractile fibres. 



IN all the Coelenterata, except the Ctenophora, the contractile 

 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 Kleinen- 

 berg, are known as myo-epithelial cells. Their contractile parts 



may either be striated (fig. 376) or 

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

 stances the epithelial part of the cell 

 may nearly abort, its nucleus alone re- 

 maining (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 Choetopoda, the Gephyrea, 

 the Chsetognatha, 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 muscular system of other 

 types is still very obscure. The muscles may arise in the embryo from 

 amoeboid or indifferent cells, and the Hertwigs 2 hold that in many of 

 these instances the muscles have also phylogenetically taken their 

 origin from indifferent connective tissue cells. The subject is how- 

 ever beset with very serious difficulties, and to discuss it here would 

 carry me too far into the region of pure histology. 



1 If recent statements of Metschnikoff are to be trusted, the Ecbinodermata 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, accord- 

 ing to Metschnikoff, only to the cutis, while the same naturalist states the epithelial 

 cells of the vasoperitoneal vesicles are provided with muscular tails. 



2 0. and R. Hertwig, Die Ccclomtheorie. Jena, 1881. 



