28 THE TROPISMS 
lates and in spermatozoids and swarm spores it has been 
supposed that there is a direct turning toward the chemical. 
This may occur in some forms and more especiallly the 
symmetrical ones, but it has not yet been established. 
Analogy with the effect of light on Euglena which may call 
forth either the motor reflex or a gradual swerving of the 
organism toward the stimulated side makes the existence 
of a parallel response to chemical stimulation more or 
less probable. 
The ccelenterates show scarcely any behavior which can be 
described as chemotactic although reaction to chemicals is of 
course general. If a Hydra is locally stimulated by a strong 
chemical which is allowed to diffuse against the body from 
a fine capillary tube there is contraction of the muscles in the 
stimulated area causing the body to bend toward the chem- 
ical. This is certainly not a teleological response; rather 
the reverse of one. A still stronger stimulus however will 
produce a general contraction of the body which enables the 
animal to avoid more serious injury. In planarians both 
positive and negative reactions may be induced. The com- 
mon fresh-water Planaria maculata may be made to follow a 
piece of meat around in any desired direction. Thesubstances 
diffusing from the meat seem to stimulate the worm to turn 
its head directly toward the food. Strong chemicals, on 
the other hand, cause it to turn directly away. Earth- 
worms, according to Darwin, find food that is buried under 
the earth through the sense of smell. Disagreeable sub- 
stances cause contraction and writhing about, which may 
bring the worm out of the region of the offending stimulus. 
A strong chemical applied to one side of the body may cause 
the animal to turn toward the other side, but the avoidance 
of such stimuli is mainly effected by general and random 
movements. 
