NATURAL HISTORY OF ONCHIDIUM 
471 
orienting illumination, but the rate of progression, with orien- 
tation established, relatively independent of this intensity. 
In agreement with this conception, we have found differences^ 
in the speed with which a dark-adapted Onchidium is oriented 
by light of different effective intensities, but with very slight 
TABLE 2 
Showing the result of one experiment in ivhich five dark-adapted Onchidia ivere ori- 
ented and allowed to creep in illuminated fields of three intensities of horizontal 
light. The different intensities were secured by placing the center of the obser- 
vation stage at distances of 12, 24, and 36 inches , respectively , from a water-screened 
oil-lamp. At the beginning of each test the light impinged upon one side of the 
animal, so that 'complete orientation' required a turning through 90°. When 
orientation had been completed, the rate of creeping over a 6-cm. stretch ivas 
measured in the same illumination 
II 
III 
ANIMAL 
Orientation 
Locomotion 
Orientation 
Locomotion 
Orientation 
Locomotion 
1 
5.1 
7.3 
2.5 
7.0 
1.2 
5.2 
2 
4.8 
4.1 
3.7 
5.3 
1.1 
6.1 
3 
5.5 
5.2 
3.1 
6.1 
0.9 
4.3 
4 
4.2 
6.1 
5.3 
4.8 
1.1 
6.0 
5 
5.6 
4.9 
2.9 
5.0 
0.8 
4.5 
Means 
5.0 
5.5 
3.5 
5.6 
1.0 
5.2 
Orient. 
Locom. 
100 
sec. for complete orientation, 
cm. 
If the rate of orientation were directly proportional to the light intensity, 
rates under I, II, III, should be in the proportion 9:4:1; actually they are as 
5.0:3.5:1. More extensive tests might provide a more complete agreement. 
The rate of progression is practically independent of the light intensity. 
differences in the speeds of progression under different intensities 
of light, provided the intensity be sufficient to keep the snail 
moving continuously. There is undoubtedly, as in Chiton 
(Arey and Crozier, '19), some correlation between speed of pro- 
' In making this statement we must for the present regretfully rely mainly 
upon the results of qualitative experiments. When this work was being done 
at the Dyer Island laboratory no electric light or other suitable light source was 
available. Kerosene lamps did not afford light of adequate intensity. 
