282 STUDIES IN GENERAL PHYSIOLOGY 
of heliotropism upon the temperature and concentration of 
the sea-water in marine animals which are far removed in 
the animal scale from the Annelids, namely, Copepods. I 
worked on a series of Copepods which were always collected 
by the tow net, a number of which, however, are not yet 
classified. The species which was found in largest numbers 
is, according to Dr. Bumpus, probably Temora longicornis. 
The freshly caught Copepods were not negatively heliotropic 
at first like the Polygordius larve, but positively heliotropic; 
the majority of these, however, soon became negatively 
heliotropic. Jarring made them positively heliotropic tem- 
porarily. They were much less resistant than Polygordius 
larve. An increase in temperature made the positively 
heliotropic Copepods negatively heliotropic, and increased 
the negativeness of the Copepods already negatively helio- 
tropic. A lowering of the temperature made the negatively 
heliotropic Copepods positive and increased the positiveness 
of the positively heliotropic Copepods. An increase in the 
concentration of the sea-water increased the positiveness, 
and a decrease in concentration increased the negativeness, 
of the Copepods. A few examples may serve to illustrate 
these facts. At +22° C. the Copepods were positively 
heliotropic. In the course of five minutes the temperature 
was raised to 26° C., when the majority became negative. 
I continued to raise the temperature to 32° C., but the ani- 
mals remained negative. I then cooled the water. The 
animals became positive when only 20° C. was reached, and 
remained so while the temperature continued to fall. The 
change in the temperature seems to be the important cireum- 
stance in Copepods which changes the sense of heliotropism. 
In another experiment the Copepods were negative at 24° C. 
I cooled the vessel rapidly so that the temperature fell to 
21° C.; at this point a part of the animals already became 
positively heliotropic. At a temperature of 7° ©. all had 
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