EVOLUTION OF THE NERVOUS SYSTEM 291 



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 them- 

 selves, some being apparently determined for the reception of mechanical 



FIG. 134. Diagram of subepithelial plexus of nerve fibres and nerve-cells, communicat. 

 ing on the one side with the sensory epithelium, and on the other side with the sub- 

 umbrellar sheet of muscle fibres. (After BETHE.) 



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 direc- 

 tion of the weight of which the cells are able to react to changes in the position 

 of the animal in space. In the jelly-fish therefore the nervous or reactive 

 system has already acquired a considerable degree of differentiation. 



FIG. 135. Figure of a jelly-fish in which all the marginal bodies except one have 

 been removed, and which has been incised in various directions so as to divide 

 the nerve ring and all the ' long paths,' so that only the diffuse nerve network 

 remains functional. (ROMANES.) 



We may study the behaviour of a more primitive system if we remove 

 the special sense-organs of the medusa by cutting off the whole of the marginal 

 ring with its contained marginal bodies (Fig. 135). We have then a layer 



