44 ANIMAL AGGREGATIONS 



and the bunch would at last become consolidated by slight move- 

 ments on the part of those on the periphery. Partially successful 

 attempts were made to control the place of bunch formation on a 

 uniform field by gluing a recently killed isopod to the substratum. 



When a drop of water was introduced on a dry background, the 

 isopods tended to occupy all of that favorable location regardless of 

 whether or not they were in contact. The bunching in close physical 

 contact came later, and might take place as a thigmotropic reaction, 

 perhaps modified by chemical stimuli, or might have been condi- 

 tioned by the drying of the small moistened region. 



Similarly, detailed studies have been made on the bunching be- 

 havior of the ophiurid starfish, Ophioderma brevispina Say (Allee, 

 1927), which lives in the eelgrass along the Atlantic Coast of North 

 America from Cape Cod southward. Individuals of this species have 

 not been found in physical contact in nature during the summer and 

 late autumn, but the collectors for the Marine Biological Laboratory 

 report that large numbers may aggregate in late November and 

 December. In the laboratory the tendency to collect in bunches dis- 

 appears as conditions approach those obtaining in nature. Thus, 

 bunches were absent or rare when eelgrass was present in approxi- 

 mately natural condition. These relations held even under the tem- 

 peratures of about 10° C. obtaining in laboratory aquaria in late 

 December. 



When, however, the Ophioderma were placed in bare containers, 

 bunches formed within a short time. The speed of formation was 

 retarded by the slower movement accompanying low temperatures 

 and dim illumination. The efi"ect of changes in illumination are 

 shown by the following example : With a constant temperature near 

 20° C. one lot formed a compact aggregation in from i to 10 minutes 

 in different trials in direct sunlight; in from 14 to 25 minutes in 

 diffuse light, and in from 27 to 56 minutes in complete darkness. 



Detailed observations of the method of formation of a large num- 

 ber of these aggregations made under a variety of conditions show 

 that the collections occur in the less illuminated part of the container 

 when there is a difference in light intensity. When conditions are 

 uniform, the starfish cluster about one of the least active individu- 



