194 ANIMAL AGGREGATIONS 



We have, then, indications that the provision of an opportunity 

 for physical contact with the Hfeless glass rods produces effects 

 similar to those given by the bunching of live starfishes. Obviously, 

 more work is needed at this point before we can come to a definite 

 conclusion. Other preliminary experiments indicate that the pres- 

 ence of irregular heaps of paraffined glass rods or of parafiined 

 rubber tubing tends to prevent autotomy of arms of individuals as 

 compared with other starfishes isolated into plain dishes. 



NATURAL AGGREGATIONS OF ASELLUS 



The fortunate discovery of a group of gigantic aggregations of 

 water isopods, Asellus communis, gave opportunity for further res- 

 piration experiments on these under natural conditions. An account 

 of some aspects of these aggregations has already been given, de- 

 scribing the methods of formation of such groups. For our present 

 purposes it is necessary to remember that these aggregations oc- 

 curred at the downstream end of culverts and on the downstream 

 side of an overflow across a sand roadway which divided an exten- 

 sive cat-tail swamp at the head waters of Dune Creek, in the Indiana 

 dunes region. At the lower edge of this overflow, just before it 

 widened out into the lower swamp, great masses of Asellus collected 

 in winter and early spring about willow shrubs, old cat-tails, or in 

 depressions where they might find a lodging. 



As was shown in the analysis in a preceding chapter, the forma- 

 tion of these aggregations was conditioned to a large extent, perhaps 

 completely, by the interaction of the tropisms of the individuals 

 with environmental factors. In turn, the accumulation of animals 

 was sufficient to affect decidedly the water surrounding and pene- 

 trating them. This effect was shown by the loss of 60 per cent of 

 the oxygen normally present in the stream. In one observation the 

 stream above the main bunch had 6.37 cc. of oxygen per liter, while 

 a collection from the midst of this large aggregation had but 2.61 

 cc, a loss of 3.76 cc. of oxygen. Similarly the pH of the stream was 

 lowered as much as 0.2 of a pH unit in extreme cases, and averaged 

 0.09 of a unit less than normal when all the twenty-three tests were 

 considered. These changes were brought about when the clusters 



