VoL. 2] Torrey—The Behavior of Corymorpha. 339 
ing their turgidity, of producing movements in the stem com- 
parable with those which are due to stimulation by gravity, 
though the evidence does not demonstrate that gravity itself 
affects them, directly or indirectly. But they are active, not 
passive elements, which increases the probability that they may 
be concerned in the geotropic response. 
If it be true, as seems probable, that the axial cells do govern 
the geotropie response, Corymorpha stands alone among the met- 
azoa in possessing a tropic mechanism distinet from the body 
musculature. In a recent paper, Holmes (:05) has shown that 
Ranatra exhibits uncommonly clear cut reactions to light. Here 
the tropic mechanism involves the same sense organ, the same 
musculature and to some extent at least the same nervous ele- 
ments which serve in responses to other classes of stimuli, in- 
ternal and external. Though Ranatra is an unusually favorable 
object for the analysis of the tropic mechanism, the confusion 
of various factors operating over the same lines renders an ade- 
quate analysis impossible at present. The presence of a geo- 
tropic mechanism distinct from the musculature in Corymorpha 
simplifies the problem. And I am led to believe, further, that 
the simply organized nervous system of the hydroid need play 
no part in the geotropie reaction. The axial endoderm cells 
probably change their volume under a given tension which is 
apphed directly to them. Each cell may be considered a unit, 
not necessarily dependent in its actions upon its neighbors, act- 
ing with them only in so far as they may be subjected simulta- 
neously to similar stimulation. 
The response of Corymorpha to gravity is strikingly similar 
to the negative geotropism of the caulicles of plant seedlings, 
which is unquestionably dependent upon volume changes in the 
stem cells. The latter are again strikingly similar in structure 
to the axial cells of Corymorpha. There appears to be in the 
hydroid mechanism no more opportunity for a pleasure-pain 
type of reaction than among the plants; and there is no sign 
of it in the geotropic movements of the stem. Neither are there 
signs of trials and errors. The movement of the stem is very 
eradual, very definite, very direct. I know of no animal which 
