HELIOTROPISM OF ANIMALS 719 
my knowledge, no observations on this subject have as yet 
been made on animals. 
Planarians and leeches afford an example of the differ- 
ence between dorsal and ventral irritability. In leeches the 
ventral surface, which has no eyes, is more sensitive to light 
than the dorsal surface. It has already been said that this 
animal leaves the dorsal side of its aboral pole exposed to the 
light, if only its head is protected from the light. Such 
animals stick to the side of a beaker, so that their ventral 
surfaces, which carry the suckers, are directed outward. If 
diffuse light of a sufficient intensity falls upon the ventral 
surfaces of the animals, most of them leave the window and 
move to the room side. The animals then turn their dorsal 
surfaces to the light. 
Tn this case, as in all the others, only the more refrangible 
rays are chiefly active. When the animals are covered with 
red glass, orientation does not follow, or only after some time. 
If blue glass is held over them, the orientation takes place 
just as in diffuse daylight. 
The difference between the irritability of the ventral and 
the dorsal surfaces of dorsiventral animals is therefore com- 
parable with that between the oral and the aboral poles. 
3. The dependence of the irritability of a dorsiventral 
animal on the symmetry of its body must yet be discussed. 
Those elements of the body of a dorsiventral animal which 
occupy symmetrical positions with reference to the median 
plane have equal irritabilities. 
The facts which prove this are to such an extent objects 
of daily experience that a brief allusion to them will suffice. 
Tf a touch on one side of the animal calls forth a movement 
to the left, then the same stimulus applied to the opposite 
symmetrical spot on the body will cause the same amount of 
movement to the right. An object appearing in the right 
field of vision causes the same amount of movement as one 
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