284 STUDIES IN GENERAL PHYSIOLOGY 



lates. Chromalina approaches the source of light at 20. 

 but flees from it at 5 C. The sense of geotropism is also 

 altered in an entirely analogous way, and we shall see in 

 the next section that a similar result is also obtained in 

 Polygordius larvae. 



IV. DIFFERENCES IN THE MANNER OF LOCOMOTION BETWEEN 

 POSITIVELY AND NEGATIVELY HELIOTROPIC ANIMALS 



Having seen how certain positively heliotropic animals can 

 be made negatively heliotropic, let us ask whether another 

 difference, independent of the direction, can be discovered 

 in the movement of positively and negatively heliotropic 

 animals. Such a question can, of course, be answered only 

 in animals which can be studied both in the negatively and 

 positively heliotropic condition. 



The larvae of Limulus polyphemus are positively helio- 

 tropic immediately after hatching from the egg. Later they 

 are negatively heliotropic. The animals can creep as well 

 as swim in all stages of their development. In fact, one can 

 observe the animals executing both forms of movement in 

 every stage of development. In their heliotropic move- 

 ments there is, however, a typical and constant difference: 

 the positively heliotropic movements are always carried out 

 by swimming, the negatively heliotropic by crawling, mo- 

 tions. The swimming movements are easy and graceful ; the 

 walking movements, clumsy. I believed at first that this 

 difference in the manner of movement was dependent chiefly 

 upon the fact that the rays of light fell into the vessel from 

 above and without, and that in consequence the positively 

 heliotropic animals were attracted to the upper surface. I 

 do not believe, however, that this is a complete explanation. 

 I had a chance to convince myself in the larvae of Polygordius 

 that this difference in the manner of movement between 

 positively and negatively heliotropic animals can exist inde- 



