42 ANIMAL AGGREGATIONS 



Once oriented so, the animals swim toward the cathode; if the 

 current is reversed, a reversal is caused in the orientation and loco- 

 motion of the animals. Many similar cases of forced orientation and 

 locomotion under the influence of the galvanic current are to be 

 found in the literature; in certain cases the animals move to the 

 anode rather than to the cathode. A general summary of galvano- 

 tropic reactions has been given by Loeb (1918). 



Similar forced movements which lead to aggregations under favor- 

 able conditions are given in response to other stimuli, particularly 

 those of chemicals and of gravity. They are not given by all members 

 of the animal kingdom, and are more likely to be exhibited by those 

 animals which, like the insects, have a disproportionate development 

 of the sensory system in comparison with the central nervous sys- 

 tem, so that the animal becomes the creature of its sensation (Ken- 

 nedy, 1927). 



RANDOM MOVEMENTS 



On the other hand, animals may congregate as a result of a series 

 of reactions, which suggest the method described by Jennings (1906) 

 as "trial and error" or, as Holmes (1905) has put it, by "the selec- 

 tion of random movements." The classic case is that originally given 

 by Jennings, of Paramecia collecting in the more-acid portion of the 

 water they occupy. This reaction is in part, at least, a trap reaction, 

 in that the animals do not react upon entering the more-acid region, 

 but respond by the characteristic avoiding reflexes when they come 

 in contact with less-acid water, and hence are caught in the region 

 of higher acidity (Johnson, 1929). This reaction by Paramecia is so 

 well known as to have been diagramed in all the current textbooks of 

 zoology. It is worth emphasizing that such a method of formation 

 of an aggregation, while less spectacular, is not necessarily less mech- 

 anistic than is the type of reaction given by Arenicola larvae when 

 they collect under the influence of directive light stimulus. It is also 

 of interest to us that, as the Paramecia aggregate, the carbon dioxide 

 given off as a result of their normal metabolic activities tends to keep 

 the region more acid and thus the aggregation tends to perpetuate 

 itself. 



When there is a limited space available, or a limited amount of 



