Fig. 237. Isolated cells belonging to the upper nerve-ring of Carmarina hastata. (After O. and R. Hertwig.)
A. Neuro-epithelial sense-cell. c. sense-hair.
B. Transitional cell between a neuro-epithelial cell and a ganglion-cell.
(3) These processes gave rise to a subepithelial nervous plexus, in which ganglion-cells, formed from sense-cells which travelled inwards and lost their epithelial character ([fig. 237] B), soon formed an important part.
(4) Local differentiations of the nervous network, which was no doubt distributed over the whole body, took place partly in the formation of organs of special sense, and partly in other ways, and such differentiations gave rise to a central nervous system. The central nervous system was at first continuous with the epidermis, but became separated from it and travelled inwards.
(5) Nerves, such as we find them in the higher types, originated from special differentiations of the nervous network, radiating from the parts of the central nervous system.
The following points amongst others are still very obscure:—
(1) The steps by which the protoplasmic processes from the primitive epidermic cells became united together so as to form a network of nerve-fibres, placing the various parts of the body in nervous communication.
(2) The process by which nerves became connected with muscles, so that a stimulus received by a nerve-cell could be communicated to and cause a contraction in a muscle.
It is probable, as stated in the above summary, that the nervous network took its origin from processes of the sense-cells. The processes of the different cells probably first met and then fused together, and, becoming more arborescent, finally gave rise to a complicated network.
