1 82 JAX B. DEMBOWSKI. 



We come to our first question: on what conditions depends 

 the spot at which the animal starts digging? The spot is not 

 quite a random one as some rules seem to hold. Practically in all 

 cases the hole lies close to the wall of the jar and at least very 

 often the wall directed towards the light is preferred. The 

 answer is a simple one. The crab is both positively phototactic 

 and thigmotactic. During its struggling for escape Uca remains 

 at the most lighted spot of the wall; there it quiets down and 

 there it begins to dig. This explanation may be correct, but it 

 leads to further considerations. Phototaxis means of course an 

 overwhelming reaction towards the light, and only in this sense 

 it has a definite meaning. We are also positively phototactic 

 and we do not like to remain in the dark. Nevertheless nobody 

 would explain the reactions of man on this basis as our phototaxis 

 is checked by very many other reactions. Such an explanation, 

 however true in some cases, would be one-sided and rather poor. 

 But the same holds also for the crab. The animal is positively 

 phototactic, and yet it digs a burrow which conducts it away 

 from the light. During the burrowing Uca repeatedly comes out 

 of the hole towards the light and it enters the burrow which is 

 dark. Is then the phototaxis changing with every minute? In 

 the jar the crab keeps usually close to the wall during the whole 

 work and the whole burrow may be fully exposed to light, which 

 does not disturb the animal in any way. After having finished 

 the work and after having filled the burrow almost entirely with 

 sand, as described below, the crab remains at the bottom of the 

 jar, in the end-chamber, which is often fully lighted, and it may 

 remain there motionless for days. We can compel the fiddler to 

 dig in the middle of the jar making there a shallow hole and 

 driving the crab into it. But as the burrow is oblique the 

 animal soon reaches the wall of the jar at some particular spot 

 and it makes an end-chamber there, being by no means disturbed 

 by the light. W T e must consider that a similar situation never 

 occurs under normal life conditions of Uca as the hole always 

 leads to the dark. The natural burrows have their openings 

 just as often directed to the south as to the north. All the 

 evidences indicate very strongly that under fairly normal condi- 

 tions of life Uca is insensible to light, or, to say it more correctly, 



