EVOLUTION OF THE NERVOUS SYSTEM 



327 



would act as relays of force, picking up the excitations arriving 

 from the undifferentiated sensory cells, and sending them on with 

 increased vigour along the nerve network. In such a manner a stimulus 

 applied at one point could be sent on in successive relays from cell to 

 cell throughout the whole reactive tissue on the surface of the body. 

 We cannot point to any particular animal as presenting instances 

 of either of the two types of elementary nervous system just described. 

 If such exist, they have not yet been investigated, or the undifferen- 

 tiated character of their nervous tissues has thwarted the efforts of 

 zoologists to display their specific characters by staining reagents. 

 In the lowest definite nervous system with which we are acquainted, 

 namely, that of the jelly-fish, all three types of cell, the sensory cell, 

 the reactive or central cell, and the motor cell, are already developed 

 and have undergone among themselves a considerable degree of 



FIG. 135. Diagram of subepithelial plexus of nerve fibres and nerve-cells, com- 

 municating on the one side with the sensory epithelium, and on the other side 

 with the subumbrellar sheet of muscle fibres. (After BETHE.) 



differentiation. In a jelly-fish or medusa, such as aurelia or sarsia 

 (Fig. 134), the reactive tissue of the body is confined to the under - 

 surface of the so-called umbrella with the tentacles and manubrium. 

 A section through the umbrella shows a layer of epithelium con- 

 taining differentiated sense-cells, below which is a plexus or rather 

 network of fine nerve fibres with a certain number of nerve-cells 

 at the nodes of the network. From this network fibres pass more 

 deeply to end in a finer network situated among a layer of muscle 

 fibres formed, like the sensory cells, by a differentiation of the primitive 

 epithelium or epiblast (Fig. 135). Besides this diffuse nervous system, 

 there is a continuous ring of nerve fibres round the margin of the 

 umbrella, thickened at intervals by the accumulation of nerve-cells, 

 which are in close relation to special collections of sensory cells in the 

 ' marginal bodies.' These sensory cells present a differentiation 

 among themselves, some being apparently determined for the reception 

 of mechanical stimuli, others for the reception of light stimuli, while 

 others again are found in close relation with little masses of calcium 

 carbonate crystals, by the direction of the weight of which the cells 

 are able to react to changes in the position of the animal in space. 



