igS ANIMAL AGGREGATIONS 



latter consumed 52.8 cu. mm. of oxygen as against 35.1 for the ag- 

 gregated females. If the male isopods in the upper swamp have 

 about the same rate of oxygen consumption as the females there, 

 a point unfortunately not directly tested, we can reconstruct their 

 respiration history as follows: 



The isopods in the upper swamp, on being swept off their feet, 

 struggle against the current and are borne along clutching at straws 

 as they pass. Sometimes an isopod clasps another and both are 

 carried along head over heels until they are deposited in some de- 

 pression with a sandy bottom, from which they start upstream only 

 to be carried down again. Such isopods have their rate of respira- 

 tion greatly increased as a result of their exertions. If they are 

 carried on down so that they lodge among the grasses below or 

 among their fellows who have already lodged there, they again take 

 on about the same rate of respiration they had shown in the upper 

 swamp. 



COMPARISONS AND CONCLUSIONS 



Like the land isopods and the brittle starfish, Asellus is not usually 

 regarded as a social species. These isopods are frequently found in 

 greater abundance in one place than another; but outside the breed- 

 ing season, environmental analyses usually reveal significant differ- 

 ences which may condition differential distribution; and, as we have 

 seen, these natural aggregations originate largely through the reac- 

 tions of individuals to their environment, rather than through social 

 impulses. Great aggregations of these animals in nature have hith- 

 erto escaped notice; and, so far as I am aware, no other great natural 

 aggregation at the low level of group organization here existing has 

 been analyzed to find the physiological effect of aggregation upon 

 the aggregants. 



The laboratory experience outlined* above had previously shown 

 that, conditions being otherwise favorable, land isopods and the 

 brittle starfish, Ophioderma brevispina, have their rate of oxygen 

 consumption increased as a first effect of aggregation. Later the 

 rate of oxygen consumption is lowered as compared with individuals 

 isolated under similar conditions. The respiration relations were 

 not obtained at the beginning of the aggregations in the case of the 



