AGGREGATIONS OF ANIMALS 103 



The collections which show the greatest group 

 organization belong in the last class. 



Animals that collect in numbers in limited re- 

 gions on account of their response to physical 

 forces of the environment may do so because 

 their own physiological processes in the presence 

 of some stimulus such as light, cause them to make 

 certain forced movements which result in aggre- 

 gations forming as near the source of the stimulus 

 (light) as the animals can get. This type of 

 reaction is commonly called a tropism; in be- 

 having so the animals orient and move as if 

 they were giving a reflex action of the entire 

 body. One of the best examples of this sort of 

 behavior in nature is furnished by the minute 

 freely swimming larvae of the Arenicola worm of 

 our New England coasts. 



The eggs are laid in a jelly-like mass at the 

 mouth of a burrow on a sandy tide flat. The 

 newly hatched larvae are positive in their reac- 

 tion to light and move to the surface where they 

 collect in great masses unless scattered by waves 

 or tides. In the laboratory, their reaction can 

 be controlled by pencils of light, and a careful 

 study of their behavior shows that these minute 

 swimming worms turn directly toward the light 

 when it strikes one of their two eye spots and that 

 they continue to swim toward it in a long spiral. 



