REPORT OF COMMISSIONERS OF INLAND FISHERIES. 183 



certain definite relation to the direction of the rays of light, and to move 

 toward or away from source of those rays. 



But what, then, is the nature of the so-called photopathic reaction? 

 It seems probable that, as Yerkes* has suggested, a photopathic 

 reaction is one in which an organism "selects a particular intensity 

 of light and confines its movements to the region illuminated by 

 that intensity." This type of reaction constitutes what will be con- 

 sidered in the following pages as the photopathic response, whether 

 or not it is brought about through a series of phototactic reactions as 

 Yerkes believed to be the case for Daphnia. Thus, to restate, it will 

 be assumed that a photopathic reaction is one in which an organism ^ 

 without previous assumption of a body orientation to the direction of 

 the rays of light, selects a region of a certain optimal light intensity. It 

 is hoped that continued investigations may throw further light on 

 the problem of the manner in which light brings about the pro- 

 gressive orientation in one direction or another; also on the method 

 by which the reaction, here assumed to be photopathic, is produced 

 (if so) without the ])revious assumption of a body orientation to 

 the direction of the light rays. 



It seems extremely probable that the behavior of the larval and 

 early adolescent stages of the lobster is determined in a large measure 

 by both kinds of response. Indeed the relation between phototaxis 

 and photopathy appears to be a very intimate one, and we can not 

 study one without taking the other into careful consideration. 

 Therefore in the following pages it will be found advantageous to 

 examine both the photopathic and the phototactic reactions in 

 more or less close connection, that we may understand, not only their 

 individual and separate, but also their mutual and combined, effect 

 upon the behavior of the larval lobsters. 



Method and Apparatus. — The experiments which are to be reported 

 involved the use of the apparatus mentioned below: (1) Cylindrical 

 museum jars, 20 to 25 centimeters in diameter. These were used in 



♦Reactions of Daphnia pulee to Light and Heat, reprinted from the Mark Anniversary 

 Volume, 1903, Article XVIII, pp. 359-377. 



