414 The American Naturalist. (Mey, - 
movements are affected by the presence of chlorophyll; on account 
of their slowness, the movements may be accurately followed step 
by step. 
Although the observations recorded in the following pages 
have occupied my attention at intervals for several years, they are 
still far from exhaustive, and I offer them only as a beginning. 
They indicate, however, that the purpose of the creeping move- 
ments and the stimuli that call them forth have not hitherto 
received any satisfactory explanation, and that a number of very 
interesting physiological questions connected with them have in 
consequence been overlooked. Since the heliotropic movements 
are complicated by-other actions, I will first describe the general 
character of the movements as a whole. 
Il. General Character of the Movements—Marshall has given 
a very good account of the mode of locomotion of Hydra, 
though he makes no attempt at an accurate analysis of the 
movements, and does not mention heliotropism. I shall there- 
fore treat only of the general character of the movements. The 
following account applies both to H. viridis and to H. fusca, 
unless otherwise stated. Ina light of moderate intensity (ina 
north room) the animals, after wandering more or less irregularly 
about, gradually collect on the side turned towards the window, 
usually not far from the surface of the water, though here and 
there a straggler lags in the background or along the sides of 
the aquarium. The movements then become less active; the 
animals may remain for a considerable time with only slight 
changes of position, and, if the food be abundant, rapidly increase 
in number by growth and budding. It appears, therefore, that in 
moderate daylight Hydra is positively heliotropic, and its | 
behavior is the same with lamplight, even if it be of very p 
intensity. If the intensity of the light be increased, a points 
ultimately reached at which the action is reversed and the animals 
move away from the light (z. e., the heliotropism becomes negative), 
though this action is less striking in its resu $ 
movement, since the animals do not collect on the side bee S 
to the light, but move into the shadow of leaves, etc, OF 3 z 
3 Zeitschrift für Wiss. Zoologie, XXXVII., 1882. 
its than the advance 
