DAILY MIGRATIONS OF COPEPODS. 
117 
come their negative geotropism and the positive phototropism of the females might 
be expected to bring them to the surface of the sea in the day instead of in the night, 
as is their habit. 
The positive phototropism of the females is, however, so strong that the question 
arises here, as it did in connection with geotropism, Are there not perhaps natural 
means of inverting this? Certainly the conditions in the laboratory did not reproduce 
those in the outer sea water, for in a large jar kept on the laboratory table the female 
Labidocerje kept persistently near the top of the water day and night, although in 
another large jar floated in the outer basin of the Fish Commission dock, and thus 
freely exposed to the elements, the females made regular migrations, being close to 
the surface in the night and at the bottom in the day. 
Experiments were now undertaken to ascertain whether the sense of the photo- 
tropism of the females or possibly even of the males could be changed. Loeb (1893, 
p. 96) found that for the copepod Temora longicornis an increase of temperature 
changed positively phototropic individuals into negatively phototropic ones and inten- 
sified the negativity of negative specimens, and that a decrease of temperature changed 
negative into positive individuals and increased the positivity of positive ones. 
Holmes (1901) found that in certain amphipods an increase of temperature certainly 
hastened and perhaps induced the positive condition. Yerkes (1900, p. 117) was unable 
to change the sense of the light reactions of Daphnia and of Cypris by temperature 
differences. All efforts to change the phototropism of Labidocerse by changing the 
temperature of the sea water were without avail. Both males and females taken from 
water at 23° C. and placed in waters at 10°, 15°, 30°, and 35° C. remained, so far as 
their phototropism was concerned, unchanged. 
Density differences were next tried. Loeb (1893, .p. 97) had shown that in Temora 
dilution of the sea water made positive individuals negative, concentration the reverse, 
but in Labidocera, animals taken from sea water of a specific gravity 1.025 and placed 
in waters of 1.050, 1.035, 1.020, 1.015, 1.010, 1.005, and 1.000 specific gravity showed 
no phototropic changes. 
Loeb (1893, p. 96) noticed that when Temora was first caught, it was often posi- 
tively phototropic, though under ordi nary circumstances it was negatively so. Shaki ng 
made the animals temporarily positive, and probably explains the peculiarity just 
mentioned. Towle (1900), by. a most painstaking series of experiments, showed that 
Cypridopsis was usually negatively phototropic, but that contact with a pipette and 
other slight mechanical stimulations were sufficient to make it temporarily positive. 
Conditions parallel with those in Temora and in Cypridopsis were often found in 
Labidocera. Females are ordinarily positively phototropic, but after having been 
several times vigorously ejected from a pipette into sea water, the majority of them 
become temporarily negative. 
The following laboratory record will show clearly the nature of this change. 
Aug. 20 (1901), 7.35 p. m. Five positively phototropic females were ejected, each one three times, 
from a pipette into sea water. Two remained positive and three became negative, swimming to the 
side of the glass vessel away from the light. 
7.45 p. m. One of the three negative individuals became positive and swam with some irregularity 
to the two positive individuals on the light side of the vessel. 
7.58 p. m. The second negative individual became positive. 
9.20 p. m. The third and last negative one became positive. 
