86 THE BEHAVIOR OF LOWER ORGANISMS. 



The question as to the mechanism of the electrotactic reaction in the 

 rotifer is of interest when one compares the structure of these animals 

 with that of the ciliate infusoria. The Rotifera, in place of having cilia 

 scattered over the entire body, are furnished only with a group of 

 cilia at the anterior end. In the Ciliata it is usually possible to distin- 

 guish functionally two groups of cilia (i) the adoral cilia, about the 

 mouth and oral groove, or at the anterior end ; (2) the body cilia, 

 scattered over the body. The cilia of the rotifers correspond function- 

 ally with the adoral cilia of the Ciliata. 



Pearl (1900), Wallengren (1902-1903), and others have shown that 

 in the electrotactic reaction of the ciliates the two sets of cilia are in 

 many cases from a functional standpoint differently affected. The 

 adoral cilia react under the influence of the electric current in such a 

 way as to tend to turn the oiganism toward the aboral side; that is, 

 they tend to produce the same reaction which the organism gives in 

 response to most other stimuli, a reaction not in harmony with the 

 tropism schema. The body cilia, on the other hand, are differently 

 affected on the different sides or ends of the organism ; those on the 

 part of the body directed toward the cathode striking in one direction ; 

 those on the part directed toward the anode striking in a different 

 direction. The result is that the organism, through the action of the 

 body cilia, tends to become directly oriented in a way that is in harmony 

 with the tropism schema. (For details, see the papers cited.) In those 

 ciliates in which the body cilia are much reduced, as in the Hypotricha, 

 the turning is determined throughout by the adoral cilia, so that the 

 orientation does not take place in accordance with the tropism schema, 

 while in some others, such as in Paramecium, the influence of the body 

 cilia is predominant, and the turning is in accord with the theory of 

 tropisms. 



What conditions shall we find in the Rotifera, where the only exist- 

 ing cilia seem to agree functionally with the adoral cilia of the Ciliata? 



As we have seen, Anura3a swims as a rule in rather wide spirals, 

 swerving strongly toward the dorsal side and revolving on its long axis 

 (Fig. 25). When the electric current suddenly acts upon it the organism 

 at once turns strongly toward the dorsal side, continuing the turn until 

 its head is brought toward the cathode, toward which it swims (Fig. 

 29). In some cases, as we shall see later, several reactions are neces- 

 sary for bringing the body in line with the current, but these are as a 

 rule very quickly accomplished. 



If, while the animals are swimming toward the cathode, the current 

 is suddenly reversed, the animals again turn strongly toward the dorsal 

 side, continuing the turning until their position is reversed and the 

 heads point toward the new cathode (Fig. 29). In many cases the 



