420 The American Naturalist. [May, 
add that A. viridis is far more hardy than H. fusca, being 
` able to live for many days or weeks in foul water that would 
quickly prove fatal to the latter species. This power of endur- 
ance may be due to the liberation of oxygen through the assimi- 
lative action of the chlorophyll. 
En résumé, the movements of Hydra may be resolved into three 
actions, which, taken together, insure to the animal a supply of 
food and air. These are’(1) heliotropism, (2) aérotropism, and 
(3) detachment from the support ; and the three are so combined 
as to form on the whole a cycle. Each movement appears to be 
called forth bya particlar stimulus,—the first by light-rays, the 
second by dissolved air, the third apparently by diminished food 
supply of a certain kind. The entire series of movements is useful 
to the animal, is in large part even of vital importance, and 
at first sight gives the general impression of consciousness and 
design; yet a careful analysis of the action weakens this 
impression, and indicates that it may be regarded as a series of 
rather complex reflexes, into which the element of consciousness, 
and a fortiori intelligence, need not enter at all. 
We may perhaps push the matter a step further back. 
Granting that the heliotropism of Hydra has been acquired because 
of the similar heliotropism of Daphnia, we may next seek an ex- 
planation of the latter action. The explanation lies close at hand, 
though I have never seen it stated. There can be little doubt that 
Daphnia, like Hydra, seeks the light because it there finds the 
maximum food supply. It is well known that a large number of 
microscopic green plants possess a considerable power of loco- 
motion, and that they are positively or negatively heliotropi¢ 
according to the intensity of the light, This is true, for instance, 
of the zodpores of numerous species of fresh-water alge, of many 
desmids, and other forms. These plants form a part—probably 
an important part—of the food of Daphnia, and the animal would 
accordingly gain a great advantage by acquiring a similar helio- 
tropism. Lastly, the heliotropism of the plants is no doubt 3 
provision for placing them in the optimum position for assimilation" 
It appears, therefore, that the ultimate reason for the heliott a 
of Hydra may lie in the mode of assimilation in green plants, ai 
