In all the Cœlenterata, 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 Kleinenberg, are known as myoepithelial 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.
Fig. 375. Myo-epithelial cells of Hydra. (From Gegenbaur; after Kleinenberg.)
m. contractile fibres.
There is embryological evidence of the derivation of the voluntary muscular system of a large number of types from myoepithelial cells of this kind. The more important of these groups are the Chætopoda, the Gephyrea, the Chætognatha, the Nematoda, and the Vertebrata[242].
While there is clear evidence that the muscular system of a large number of types is composed of cells which had their origin in myoepithelial cells, the mode of evolution of the muscular system of other types is still very obscure. The muscles may arise in the embryo from amœboid or indifferent cells, and the Hertwigs[243] hold that in many of these instances the muscles have also phylogenetically taken their origin from indifferent connective-tissue cells. The subject is however beset with very serious difficulties, and to discuss it here would carry me too far into the region of pure histology.
The voluntary muscular system of the Chordata.
The muscular fibres. The muscular elements of the Chordata undoubtedly belong to the myoepithelial type. The embryonic muscle-cells are at first simple epithelial cells, but soon become spindle-shaped: part of their protoplasm becomes differentiated into longitudinally placed striated muscular fibrils, while part, enclosing the nucleus, remains indifferent, and constitutes the epithelial element of the cells. The muscular fibrils are either placed at one side of the epithelial part of the cell, or in other instances (the Lamprey, the Newt, the Sturgeon, the Rabbit) surround it. The latter arrangement is shewn for the Sturgeon in [fig. 57].
