EVOLUTION OF THE NERVOUS SYSTEM 



291 



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 mechani- 



FIG. 134. Diagram of su be pit-he lial 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.) 



cal 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. 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 diiecticns 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 



