CoNGDON, Reactions to Light. 311 



men in advance. Palaemonetes larvae swim with abdomen toward the Hght. Lyon 

 ('07) caused them to move head first tow^ards the dark by diluting the seawater. 



A considerable part of Radl's investigation, like that of Loeb and Lyon at an 

 earlier period, relates to the movements of insects and other arthropods placed 

 upon disks rotating in various planes. Some animals remain standing upon disks 

 and automatically turn their heads to maintain their orientation to the light. Some, 

 if upon a slowly moving horizontal disk, keep in such a position that they do not 

 lose their orientation to light or to the surrounding, fixed environment. The rela- 

 tion of these reactions to the explanation of light responses can best be made clear 

 after recalling a view expressed by Loeb ('07, etc.). Binocular vision, he believes, 

 is phototactic because a pair of eyes are always placed symmetrically in respect to the 

 center of the field of vision by virtue of their adjustment so that it will fall upon the 

 middle points of their retinas. Radl conceives phototaxis as a response to localized 

 stimulus resulting in symmetrical adjustment. He believes binocular vision is 

 phototactic in his sense of the term phototaxis. At the same time he acknowledges 

 that such phototaxis has two novel elements: namely, the substitution of the varied 

 field for a simple source of light, and the orientation of organs instead of the whole 

 body. In regard to the former, he admits that there has not as yet been brought 

 forward any series of phototactic reactions to fields of gradually increasing complex- 

 ity as a proof that orienting to them is essentially the same as orienting to a simple 

 source of light. Radl together with Loeb and Lyon have found that orientation 

 of eyes as well as head are shown by compensatory movements to be very common 

 in insects. So frequently does it occur that Radl is led to say that the essential 

 of arthropod phototaxis is the orientation of the eyes, and that the adjustment of 

 the body follows only at times, and is of secondary importance. 



Hadley ('06, '06a) has recently shown that young lobsters keep a constant posi- 

 tion relative to the bottom while in moving water. This is partly due to orientation 

 toward the fixed field about them. The optical portion of the process may be 

 directly compared with compensatory locomotion upon a revolving disk. Mechan- 

 ical compensatory movements due primarily to light, and resembling those of the 

 young lobster, are described by Lyon ('04) in a study of the rheotaxis of certain 

 fish. Loeb ('07a) finds that marked compensatory head movements are made by 

 the reptile Phrynosoma upon the revolving disk. His experiments show them to 

 be in part due to optical reflexes. It is evident that the compensatory movements 

 of vertebrates, in so far as they are optical in origin, have in them the qualities of 

 similar arthropod movements. If the term phototaxis may be applied to the binocu- 

 lar vision of arthropods, it also can rightly be used for vertebrates. Such a state- 

 ment needs the corollary that phototaxis probably expresses only a tithe of the 

 nervous activity involved in the binocular vision of the vertebrate. 



There has been presented a fair illustration in the variety of locomotion which 

 may result from the response to localized stimulus as described by recent authors. 

 We are therefore in a position to consider whether these different procedures have 

 anything further in common. Opposite conditions in the complexity of aligning 

 movements are illustrated by the earthworm and flatworm as compared with cer- 

 tain crustaceans, as Daphnia. Some animals also stand in contrast with Daphnia 

 because of the greater irregularity of their course to or from the light. Thus m 

 Corethra peculiarities of locomotor mechanism produce a zigzag course. In spite 

 of the great variety which these animals just mentioned show in their accuracy of 



