i82 ANIMAL AGGREGATIONS 



bunching on changes in water content as indicated by weight of the 

 animals (Allee, 1926). When the isopods were placed on a moistened 

 filter paper with maximum moisture present with which the animals 

 would remain bunched, the aggregated isopods increased in weight 

 3.8 per cent from water intake while their isolated fellows were in- 

 creasing 9.7 per cent. 



Similar experiments with bunches and single individuals placed 

 under desiccating conditions showed that in a typical experiment 

 isolated isopods lost water three times as fast as did a group of 10 

 isopods gathered into a close bunch. Further, all 10 of the bunched 

 animals in this typical experiment were alive with a loss of weight of 

 less than 16 per cent after 7.45 hours, when the last of the isolated 

 individuals was found dead. The last 2 isopods to die showed a 

 water loss at this time of about 44 per cent. Sixty per cent of the 

 isolated isopods were found dead after 4.38 hours' exposure. 



These observations show a definite survival value of a group of 

 animals even at the low level of social integration existing among 

 land isopods. It may be remarked again that, outside the breeding 

 season, as these were, isopods collect in bunches due primarily to 

 non-social, individual tropistic reactions, and that almost their only 

 social attribute is tolerance for other animals and their products 

 within a limited space. 



STARFISH AUTOTOMY 



The essentials of the situation among the ophiurid starfish are 

 similar, but the mode of expression differs (Allee, 1927). Ophiurids 

 have the practice of fragmenting their arms under certain condi- 

 tions. Early experiments showed that there is a greater tendency to 

 practice autotomy with isolated than with bunched Ophiodenna. 

 The tests to be considered were run in connection with respiration 

 studies to be described later. Two groups of 8 each were placed in 

 two large bottles each containing 8 liters of sea-water. Eight similar 

 starfishes were isolated into flasks of about i liter each. The water 

 was usually changed daily and never left longer than 48 hours. No 

 attempt was made to supply food. 



The following schedule of numerical symbols was adopted : o, per- 



