PREY-PREDATOR RECOGNITION IN THE LOWER 

 INVERTEBRATES* 



L. M. Pass A NO 

 Yale University 



Above the broad herbivorous base of the ecological pyramid of animals, 

 whether one is considering the land or sea, are the lesser numbers of car- 

 nivores. Most of these animals must be below the apex, must feed on their 

 animal prey yet in turn be fed upon by other organisms. The success of 

 these animals means that a judicious balance between recklessness and 

 care, efrective capture of food versus preventive caution in the presence of 

 enemies, has been achieved. The organism's behavior patterns are adapted 

 to meet these dual needs. The animal's morphological and physiological 

 equipment dictates the form of this behavior, its complexity, and its adapta- 

 bility. Obviously, the success of this balance can be measured by the abund- 

 ance of the group under consideration. 



Without being able to explain the physiological mechanisms that are 

 involved, nevertheless we can see in the elaborate and specialized sense 

 organs, in the many and varied motor responses, and above all in the com- 

 plexity of the central nervous systems, the physiological and morphological 

 machinery that determines the success of most carnivores. But, especially 

 to those interested in comparative physiology, a few groups of relatively 

 simple metazoan carnivores, such as many of the coelenterates and the 

 free-living flatworms, pose an immediate problem. These animals do not, 

 as a rule, possess elaborate sense organs. Their tissues, as judged by such 

 criteria as regenerative powers or specialization of function, do not appear 

 to be completely subordinate to the entire organism. Above all, as far as 

 the coelenterates are concerned, they have no central nervous system. We 

 are unable to evoke any "black box" of suitable complexity to account for 

 the behavior pattern that we can so easily observe. With the relative sim- 

 plicity of both behavior and morphological equipment, it is not surprising 

 that these organisms constitute an immediate challenge to the physiologist. 



As the title of this paper indicates, only one aspect of these analytical 

 problems will be dealt with here, the recognition and distinction of prey 

 and predator. We shall try to determine how far, in the present state of 

 our knowledge, it is possible to explain these behavior patterns in physio- 

 logical language. Let us admit at the outset that this attempt cannot be too 



* This paper is dedicated to Alexander Petrunkevitch in honor of his eightieth 

 birthday. 



[37] 



