IO6 THE BEHAVIOR OF LOWER ORGANISMS. 



what is demanded by the tropism schema, though when we come to 

 details of the behavior of the organs themselves, this theory seems 

 unsatisfactory, even in the sea urchin. 



Such organisms as the sea urchin, composed anatomically and physio- 

 logically of many parts, each acting almost as an independent animal, 

 are certainly less common than more unified animals, such as we find 

 in the Infusoria, the Rotifera, the flatworms, etc. For this reason, 

 therefore, it has seemed worth while to sum up the real relations of the 

 behavior of these organisms to the tropism theory. The unicellular 

 animals are precisely those on which the prevailing theories of tropisms 

 or taxis have by many writers* been chiefly based. With the demon- 

 stration that the behavior of these organisms (as well as of many higher 

 ones), is for the greater part inconsistent with the tropism theory, per- 

 haps a large portion of the foundation for its acceptance as a general 

 formula for the chief features in the behavior of lower animals is cut 

 from beneath it. 



In the following paper, on the part played in behavior by physio- 

 logical conditions of the organism, we shall find other, and, as it seems 

 to me, still more cogent, reasons for holding the tropism theory inade- 

 quate to account for the determining factors in the behavior of most 



lower organisms. 



SUMMARY. 



The foregoing paper consists of a review of the behavior of Ciliata, 

 Flagellata, Bacteria; of Rotatoria and certain other Metazoa, so far 

 as known from exact observation of their actions when stimulated, 

 with a view to determining how far the prevailing theory of tropisms 

 aids us in understanding the behavior of lower organisms. 



The following are considered the essential points in the prevailing 

 theory of tropisms : (i) That orientation is the primary factor in deter- 

 mining the movements of organisms into or out of certain regions, or 

 their collection in or avoidance of certain regions ; (2) that the action 

 of the stimulus is directly upon the motor organs of that part of the 

 organism upon which the stimulus impinges, thus giving rise to changes 

 in the state of contraction, which produce orientation. 



The reactions of the organisms above named are then reviewed to 

 determine in how far there is agreement with these essential points in 

 the theory of tropisms. The following are pointed out : 



The reactions to mechanical stimuli, to chemicals, to heat and cold, 

 and to variations in osmotic pressure have been described in detail, and 

 it is found that orientation is not a primary nor a striking factor in 

 them. The response in all these cases is produced through a " motor 



*This, however, is not true of Loeb. 



