120 JELLY-FISH, STAR-FISH, 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 effected by means of intercom- 
municating fibres, the directions of which 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 ee 
by the consideration that it is the only one avail- 
able, is confirmed by the fact that the stimuli which 
