In sea-anemones (Hertwigs, No. [321]) there are, for instance, no organs of special sense, and no definite central nervous system. There are, however, scattered throughout the skin, and also throughout the lining of the digestive tract, a number of specially modified epithelial cells, which are no doubt delicate organs of sense. They are provided at their free extremity with a long hair, and are prolonged on their inner side into fine processes which penetrate into the deeper part of the epithelial layer of the skin or digestive wall. They eventually join a fine network of protoplasmic fibres which forms a special layer immediately within the epithelium. The fibres of this network are no doubt essentially nervous. In addition to fibres there are, moreover, present in the network cells of the same character as the multipolar ganglion-cells in the nervous system of Vertebrates, and some of these cells are characterised by sending a process into the superjacent epithelium. Such cells are obviously intermediate between neuro-epithelial cells and ganglion-cells; and it is probable that the nerve-cells are, in fact, sense-cells which have travelled inwards and lost their epithelial character.
Fig. 236. Neuro-epithelial sense-cells of Aurelia aurita. (From Lankester; after Schäfer.)
In the Craspedote Medusæ (Hertwigs, No. [320]) the differentiation of the nervous system is carried somewhat further. There is here a definite double ring, placed at the insertion of the velum, and usually connected with sense-organs. The two parts of the ring belong respectively to the epithelial layers on the upper and lower surfaces of the velum, and are not separated from these layers; they are formed of fine nerve-fibres and ganglion-cells. The epithelium above the nerve rings contains sense-cells ([fig. 237]) with a stiff hair at their free extremity, and a nervous prolongation at the opposite end, which joins the nerve-fibres of the ring. Between such cells and true ganglion-cells an intermediate type of cell has been found ([fig. 237] B) which sends a process upwards amongst the epithelial cells, but does not reach the surface. Such cells, as the Hertwigs have pointed out, are clearly sense-cells partially transformed into ganglion-cells.
A still higher type of nervous system has been met with amongst some primitive Nemertines (Hubrecht, No. [323]), consisting of a pair of large cephalic ganglia, and two well-developed lateral ganglionic cords placed close beneath the epidermis. These cords, instead of giving off definite nerves, as in animals with a fully differentiated nervous system, are connected with a continuous subdermal nervous plexus.
The features of the embryology and the anatomy of the nervous system, to which attention has just been called, point to the following general conclusions as to the evolution of the nervous system.
(1) The nervous system of the higher Metazoa appears to have been evolved in the course of a long series of generations from a differentiation of some of the superficial epithelial cells of the body, though it is possible that some parts of the system may have been formed by a differentiation of the alimentary epithelium.
(2) An early feature in the differentiation consisted in the growth of a series of delicate processes of the inner ends of certain epithelial cells, which became at the same time especially differentiated as sense-cells ([figs. 236] and [237]).