REGULATION OF REACTIONS 285 



during another. Thus we find the plumules of many of the 

 grasses (gramineae) very sensitive to light during the early 

 stages of development and not at all later. Fly larvae are 

 strongly negative, but the imagos are positive. Loeb 

 (1906, p. 134) found that the nauplii of Balanus are positive 

 when they leave the egg, but that they become negative 

 soon afterward. I have observed similar changes in reac- 

 tions in the larvae of Arenicola, Limulus, and Hydroides 

 dianthus, in various larvae of crabs and in the medusae of 

 Bougainvillea. There is a striking peculiarity connected 

 with the change in the sense or reaction of Limulus larvae. 

 When these animals proceed toward the source of light 

 they always swim, but when they proceed from it they 

 always crawl. They usually swim most of the time 

 when they are young and are positive, but when they get 

 older they nearly always crawl on the bottom and are 

 negative. If specimens which are crawling from the 

 source of light are agitated until they swim they proceed 

 toward the light, but as soon as they touch the bottom 

 and begin to crawl they go away from the light. I have 

 repeatedly seen specimens in a glass dish swim toward the 

 source of light against the side of the dish, sink to the bot- 

 tom and crawl from the light several centimeters, then 

 start up again and swim toward the light, and so on, 

 repeating the process many times. Contact seems to 

 have something to do with the sense of reaction here, but 

 the fundamental causes of the changes are no doubt rooted 

 in the developmental changes in the organism associated 

 with its habits. In nearly all of the species mentioned 

 above the changes in the sense of the reactions are un- 

 doubtedly adaptive. When the larvae first leave the egg 

 they are strongly positive, and swim out in various direc- 

 tions from the site of their birth so as to become widely 

 scattered. Later, when the developmental processes pre- 

 pare them for sedentary life, they become negative and 

 consequently go to the bottom, where they become at- 

 tached or burrow in the mud. It is not probable that the 



