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 larve. 
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 heliotropie condition. 
The larve of Limulus polyphemus are positively helio- 
tropic immediately after hatching from the egg. Later they 
are negatively heliotropic. he 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. 
Thad a chance to convince myself in the larve of Polygordius 
that this difference in the manner of movement between 
positively and negatively heliotropic animals can exist inde- 
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