1891.] The Heliotropism of Hydra. ` 429 
TABLE IIIl.—Aydra viridis. 
EN E E Il. HE ORE: Wa VEL VIL 
ARRANGEMENT. . .... G merg y R 
March 24, 5.30 P.M. I o 6 2 8 2 2 
*: 8.30 P.M I I 7 6 5 5 I 
Mo, 945 AN. o o 14 o o 7 o 
REARRANGED pe o Y B R 
March 25, 2.00 P.M o 3 4 5 5 4 o 
H + 26, 5.00PM ° 5 I 6 6 4 o 
©. 20- 5.00 P.M o 7 > 3 6 6 o 
way, 4.00 P.M o 3 o 4 9 6 2 
a 38,4200: 7M o 3 o 5 9 6 2 
; s 29, _ 3.00 P.M. o 3 o 2 I5 4 o 
REARRANGED ERER E T y R B 
March 29, 9.45 P.M. o 4 o 9 6 8 2 
-g0 9.45 AM. o 4 o 10 I 6 8 
" 31, 9.00 A.M. ce) 4 o 12 o 5 9 
REARRANGED ole Fas -B B 
March 31, 12.30 P.M. I I 7 4 6 6 4 
Ame. 1... O30 AM. s I II 2 9 4 4 
REARRANGED ee SoS i: tee Bı 
prl 2,1000AM. 5 cw o- 13 6 
=e GOP 4 2 7 to F : 
7 oe trea. 6 3 3 Ceara 6 5 
EXPLANATION OF TABLE III. 
General conditions of the experiment, as in Tables I., II., but the ani- 
mals were in a cylindrical aquarium, eight inches in diameter, and the areas 
were much smaller (colored areas, 20 by 70 mm. ; light areas, 10 by 70 mm.). 
The middle area (IV.) was turned towards the window. The end areas 
(L, VII.) would therefore tend to receive any Hydras advancing towards the 
light around the sides. 
e results show a complete avoidance of all colors except blue, and a 
marked “ preference ” for blue as compared with ordinary daylight. 
The results obtained by the use of colored glasses are con- 
firmed by tests with the actual spectrum.’ If a spectrum 
produced by passing a beam of light from an Argand lamp 
through a prism be thrown upon a group of Hydras, they show 
a very marked tendency to collect in the lower blue. It is 
*For this purpose I have used an Argand gas-burner, the light from which was passed 
; rgan ; 
first through a narrow slit, then through a biconvex lens to render the rays approximately 
Parallel, and finally through a large prism (bisulphide bottle). The spectrum thus ob- 
ium rul 
tained was projec za ed in small squares, and at 
Aa A migi im — the side of a square aquari 
