160 INTELLIGENT BEHAVIOUR 
and come to rest on the bottom. ‘Then, taking the utmost 
care not to jar the dish or breathe upon the surface of the 
water, look at the Clepsine through a low magnifying lens, and 
see what happens when the surface of the water is touched 
with the point of a needle held vertically above the animal’s 
back. If the experiment is properly carried out, it will be 
seen that the respiratory undulations (if such movements 
happen to be going on) suddenly cease, and that the animal 
slightly expands its body and hugs the glass. Wait a few 
moments until the animal, recovering its normal composure, 
again resumes its respiratory movements. ‘Then let the needle 
descend through the water until the point rests on the bottom 
of the dish at a little distance from the edge of the body. 
Again the movements will cease, and the animal will hug the 
glass with its body somewhat expanded. Now push the needle 
slowly along towards the leech, and notice as the needle comes 
almost in contact with the thin margin of the body, that the 
part nearest the needle begins to retreat slowly before it. 
This behaviour shows a surprising keenness of tactile sensi- 
bility, the least touch of the water with a needle-point being 
felt at once. . . . If its back were rubbed with a brush or the 
handle of a dissecting needle, in order to test its sensitiveness 
to touch, the appearance would probably be that of insensi- 
bility and indifference to the treatment. Closer examination, 
however, would show that the flesh of the animal was more 
rigid than usual, and that the surface was covered with 
numerous stiff, conical elevations, the dermal papille or warts, 
which are so low and blunt in the normal state of rest as to be 
scarcely visible. It would be seen that the animal, although 
motionless, was in a state of active resistance to attack... . 
Clepsine has another and entirely different method of keeping 
quiet. ‘The animal rolls itself up (head first and ventral side 
innermost) into a hard ball, outwardly passive, free to roll or 
fall whithersoever gravity or currents of water may direct 
it. . . . If by chance the animal has eggs, it will not desert 
them to escape in this way. . . . This species, then, has two 
quite distinct and peculiar ways of keeping quiet, and thus 
