The primitive relations between the nervous network and the muscular system are matters of pure speculation. The primitive muscular cells consist of epithelial cells with muscular processes (fig. 238), but the branches of the nervous network have not been traced into connection with the muscles in any Coelenterata except Ctenophora. In the higher types a continuity between nerves and muscles in the form of motorial end plates has been widely observed. Even in the case of the Coelenterata it is quite clear from Romanes' experiments that stimuli received by the nerves are capable of being transmitted to the muscles, and that there must therefore be some connection between nerves and muscles. How did this connection originate? Epithelial cells with muscular processes (fig. 238) were discovered by Kleinenberg (No. 324) in Hydra before epithelial cells with nervous processes were known, and Kleinenberg pointed out that Hydra shewed the possibility of nervous and muscular tissues existing without a central nervous system, and suggested that the epithelial part of the myoepithelial cells was a sense-organ, and that the connecting part between this and the contractile processes was a rudimentary nerve. He further supposed that in the subsequent evolution of these elements the epithelial part of the cell became a ganglion-cell, while the part connecting this with the muscular tail became prolonged so as to form a true nerve. The discovery of neuro-epithelial cells existing side by side with myoepithelial cells demonstrates that this theory must in part be abandoned, and that some other explanation must be given of the continuity between nerves and muscles. The hypothetical explanation which most obviously suggests itself is that of fusion. It seems quite possible that many of the epithelial cells of the epidermis and walls of the alimentary tract were originally provided with processes, the protoplasm of which, like that of the Protozoa, carried on the functions of nerves and muscles at the same time, and that these processes united amongst themselves into a network. Such cells would be very similar to Kleinenberg's neuro-muscular cells. By a subsequent differentiation some of the cells forming this network may have become specially contractile, the epithelial parts of the cells ceasing to have a nervous function, and other cells may have lost their contractility and become solely nervous. In this way we should get neuro-epithelial cells and myoepithelial cells both differentiated from the primitive network, and the connection between the two would also be explained. This hypothesis fits in moreover very well with the condition of the neuro-muscular system as we find it in the Coelenterata. BIBLIOGRAPHY. _Origin of the Nervous System._ (316) F. M. Balfour. "Address to the Department of Anat. and Physiol. of the British Association." 1880.