REACTIONS TO STIMULI IN CERTAIN ROTIFERA. 



In my series of " Studies on Reactions to Stimuli in Unicellular 

 Organisms" and in the foregoing papers I have set forth the reaction 

 methods of many infusoria to varied stimuli. The result has been 

 to show that the reaction method in these organisms is of a peculiar 

 character, differing radically from that required by prevailing theories 

 of the reactions of lower organisms. The essential nature of these 

 reactions, with their implications as to the character of behavior in the 

 lower organisms, will be discussed in the following papers. Before 

 I^roceeding to this discussion it is important to determine whether the 

 reaction method in the Infusoria differs radically in character from 

 that of Metazoa. For tliis purpose it seems well to select a group of 

 Metazoa whose habitat and mode of life are similar to those of the 

 Infusoria. In this way differences due primarily to the different plan 

 of structure of the two sets of organisms may perhaps be brought out 

 without the complications arising from different modes of life. 



A group of Metazoa much resembling the Infusoria in their mode 

 of life is found in the Rotatoria. As is well known, the members 

 of these two groups are usually found mingled to<j;ether. They are 

 of about the same size, and both swim about by means of cilia. So 

 great is the resemblance in general habit and in habitat that they were 

 at first classed together, all being given the name of Infusoria. As 

 we know now, however, they are really widely separated in relation- 

 ship. While the Infusoria are unicellular, the Rotifera are multicellu- 

 lar organisms of a high degree of complexity, possessing many systems 

 of organs, each composed of many cells. In particular, they have a 

 well-developed nervous system. 



A comparison of the behavior of these two groups of organisms 

 should show us, therefore, whether there are types of reaction having 

 a high degree of generality, such as is claimed for the theory of 

 tropisms — types that may give a key to the behavior of groups so 

 widely separated in relationship as the two under consideration, which 

 are representatives of the Protozoa and of Metazoa of a fairly high 

 degree of organization. 



In the present paper I can attempt to give an account of the behavior 

 of only a few free-swimming species, and that not in an exhaustive 

 manner. I hope to return to an extensive study of the behavior of this 

 interesting group, so as to develop its implications for the theory of 



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