28 THE TROPISMS 



latcs 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 its 

 course toward the stimulated side makes the existence of a 

 parallel response to chemical stimulation more or less 

 probable. 



The coelenterates 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. The substances 

 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. 



