224 ANIMAL AGGREGATIONS 



what related to Convoluta. These animals are normally subjected 

 to hypotonic sea-water when a heavy rain occurs at low tide. 



These small worms reach a length of about 5 mm. and are about 

 I mm. in width. In nature they are found in abundance on the lower 

 sides of stones in small tide pools, near or below low tide line. They 

 are not abundant in deeper water. Usually they were taken from 

 the protected sides of stones that were firmly located on a sandy 

 substratum; evidently they cannot stand the full sweep of the waves. 

 They were usually present on a given stone in considerable numbers, 

 if at all. As the water went stale in the laboratory, if there were 

 numbers of worms present they would collect on the surface film in 

 shaded areas in dense aggregations. Here, as in the field, they did 

 not occupy all the apparently optimal space. 



Appropriately safeguarded experiments showed that these worms 

 will survive exposure to equal amounts of tap-water the better (a) 

 if they are present in numbers; {b) if isolated into tap-water in 

 which other living Procerodes have previously been exposed; and 

 especially (c) if exposed in tap-water in which other Procerodes have 

 died and disintegrated in whole or in part, even when such a condi- 

 tioned medium is boiled or filtered. As is to be expected, these 

 worms live longer if the salinity of the tap-water is increased by as 

 much as 0.05 per cent above a minimum of that value. With well- 

 washed worms it is possible to demonstrate protection when the 

 tap-water contains exudates from living or dead Procerodes in which 

 the salt concentration, as measured by the amount of chlorides pres- 

 ent, is not the determining factor. The method used consists in titrat- 

 ing with N/ioo silver nitrate, using a i per cent solution of potas- 

 sium chromate as an indicator. 



The first experiments (Allee, 1928) demonstrated that the pro- 

 tection was due neither to sea-water contamination nor to the leach- 

 ing-out of electrolytes in the proportions in which they exist in sea- 

 water. They did not exclude the possibility that the protection may 

 have been given by the leaching-out of electrolytes in some other 

 proportion from that found in sea-water. Such an explanation of 

 the observed protection is the most simple and obvious one to be 

 advanced. The possibility of its operation was tested at the first 

 opportunity. 



