328 



PHYSIOLOGY 



In the jelly-fish therefore the nervous or reactive system has already 

 acquired a considerable degree of differentiation. 



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. 136). We have then a layer of contractile tissue, innervated 

 by a nerve network, and covered by a layer of epithelium containing 

 sense-cells. To this network is attached the manubrium, which 

 represents the mouth and stomach of the animal. In such a mutilated 



FIG. 136. 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.) 



jelly-fish it is easy to show that a stimulus applied to one spot on the 

 surface travels outwards from the excited spot to all parts of the 

 bell. The stimulus is propagated also to the manubrium, which in 

 some species bends in the direction of the excited spot that is to 

 say, in the direction which represents the shortest possible path 

 from the excited spot to the manubrium. This preparation rarely 

 presents any automatic activity. It may react to a constant stimulus 

 by a rhythmic series of contractions, but remains perfectly motion- 

 less in the absence of stimulus. The unmutilated jelly-fish presents 

 rhythmic contractions of its sub-umbrella tissue which are inaugurated 

 in any or all of the marginal bodies and serve to drive the animal 

 onwards through the water in which it is immersed. The rhythmic 

 contractions may be initiated, augmented, or diminished, in response 

 to stimuli of light, mechanical irritation, or changes in the position 



