120 JELLY-FISH, STAK-FLSH, AND SEA-URCHINS, 



tissue thus far resemble afferent nerves in their 

 function, we soon see that in one important par- 

 ticular they differ widely from such nerves; for 

 we have seen that, after they have been divided, 

 stimulation of their peripheral parts still continues 

 to be transmitted to their central parts, as shown 

 by the non-localizing movements of the manubrium. 

 Of course this transmission cannot take place 

 through the divided tissue-tracts themselves; and 

 hence the only hypothesis we can frame to account 

 for the fact of its occurrence is that which would 

 suppose these tissue-tracts, or afferent lines, to be 

 capable of vicarious action. Such vicarious action 

 would probably be efiected by means of intercom- 

 municating fibres, the directions of w^hich would 

 probably be various. In this way we arrive at the 

 hypothesis of the whole contractile sheet being 

 pervaded by an intimate plexus of functionally 

 differentiated tissue, the constituent elements of 

 which are capable of a vicarious action in a high 

 degree. 



Now we know from histological observation that 

 there is a plexus of nerve-fibres pervading the whole 

 expanse of the contractile sheet, and therefore we 

 may conclude that this is the tissue through which 

 the effects are produced. But, if so, we must 

 further conclude that the fibres of this nerve-plexus 

 are capable of vicarious action in the high degree 

 which I have explained. 



And this hypothesis, besides being recommended 

 by the consideration that it is the only one avail- 

 able, is confirmed by the fact that the stimuli which 



