PREY-PREDATOR RECOGNITION 41 



under consideration that visual stimuli do )iot directly guide either their 

 defensive or predaceous behavior. Rather, those forms which have photo- 

 reception appear to use this information to direct or modify overall activity 

 patterns, and to keep them within the generally restricted environmental 

 niche to which they are adapted. 



This sort of resume of the physiological mechanisms guiding such com- 

 plex organisms as annelids and mollusks seems, and is, very naive. When 

 one comes to the free-living flatworms, however, such simplifications may 

 begin to come somewhat closer to validity. Thanks largely to the classic 

 observations of Pearl ( 1902) and the careful analysis of the role of chemo- 

 reception by Koehler (1932), a considerable body of information is avail- 

 able concerning planarians. These carnivores are guided to their prey 

 mainly by chemoreceptors,. utilizing separate sensory units for weak and 

 strong stimuli (Wulzen, 1917). As in the gastropod mollusks, these units 

 appear to be the analogues of vertebrate smell and taste receptors, respec- 

 tively. If the "physiological condition" of the animal allows it, weak 

 chemical stimuli will initiate and orientate a positive gliding movement. 

 In some species this movement must be against the current as well as along 

 a chemical gradient (Koehler, 1932). Strong illumination may prevent 

 this positive response. At some point when the animal is close to the source 

 of the chemical stimulus, other chemoreceptors, generally located on the 

 proboscis or grasping organs (Redfield, 1915; Wilhelm, 1915) are 

 sufficiently stimulated to assume control of the behavior pattern, changing 

 a seeking behavior to a feeding behavior. Positive-orienting reactions may 

 also be elicited by mild tactile stimuli. The specific feeding reflex may re- 

 quire both stimuli concurrently, or else a strong chemical stimulus may be 

 sufficient. 



Pearl (1902) emphasized that positive responses to both chemical and 

 mechanical stimuli were abruptly changed to negative responses when the 

 strength of the stimuli was increased beyond some arbitrary and variable 

 point. For instance, fairly strong unilateral tactile stimulation of the an- 

 terior end of the planarian would cause a turning away from that side, 

 extension of the stimulated side. This extension, according to Pearl, is due 

 to contraction of the circular, dorsoventral, and transverse muscles. A 

 somewhat weaker stimulus, however, causes a turning towards that side, 

 due to the contraction of the longitudinal muscles. Thus, according to this 

 worker, neither "weak" nor "strong" stimuli lead to a crossing over of the 

 reflex to the opposite side. Also, both chemical and tactile stimuli lead to 

 either of these responses. Pearl's positive and negative responses, depend- 

 ing on their strength. 



In addition to these two groups of receptors on the proboscis and the 

 anterior lateral regions, there are receptors on the posterior end of these 



