44 
The Irish Naturalist. 
February, 
sensitive fringe of the head maintains a continuous wave- 
like motion, specially noticeable where what we may call 
ears " are present at the sides of the head. Occasionally, 
the animal will stop, raise the whole front end of the body 
and wave about gracefully in the water. Such movements 
are called testing movements and are specially to be observed 
under certain conditions ; e.g., before coming to rest, the 
creature " tests " the surroundings before it relaxes its 
muscles and settles down; after resting, it "tests" the 
neighbourhood before resuming progression ; all obstacles 
are " tested" to determine the nature of the impediment, 
if it is food it is appropriated, if not it is avoided ; if a 
bright beam of light is suddenly thrown on the front end 
of the animal, it " tests" it before turning aside ; and so 
on. 
As soon as an aquatic planarian appreciates any signs 
of drying, it first ' ' tests " the drop of water it is in, then 
curls up in such a way as to expose as little as possible of 
its surface, particularly protecting its sensitive head. If 
the drying is continued, it tries to " back out " exactly as 
it would if it were on the edge of a pond where evaporation 
was going on rapidly. 
Similar testing operations are exhibited when the water 
in which a planarian is placed is heated gradually. When 
the vessel is heated from one end, the animal goes to 
investigate the cause of the change taking place and proceeds 
towards the source of heat ; without imperilling his safety, 
he discovers the locality which is " too hot " for him and 
retires to cooler quarters. As the heat increases he becomes 
more and more disturbed and finally plunges about wildly 
in every direction to avoid the danger — not, however, 
until all reasonable testing movements have been tried in 
vain. An important thing to grasp is that a planarian 
moves towards all weak stimuli and "tests" them; in 
truth it has been said, the planarian ' ' proves all things, 
holding fast only to that which is good." 
Accidents arc likely to happen even in the planarian 
world and for some unavoidable reason the flat -worm may 
come to be on his back. In such a position the animal 
