STUDY ON THE HABITS OF THE CRAB. 169 



fixed after some time and it remains unaltered even if we remove 

 the case from the animal's back. I noticed repeatedly that the 

 coarse inner surface of the new case soon becomes smooth and 

 resembles closely the inner surfaces of natural cases found in 

 the sea. Very probably it depends once more on the mentioned 

 plasticity of the sponge. 



If the stone, which we put into the aquarium, was covered by 

 several distinct sponges of different size, but of somewhat 

 rounded circumference Dromia did not cut any of them, but 

 chose the most suitable one and tore it off as described above. 



It remains not fully explained why larger specimens of Dromia 

 usually carry cases on them which cover only the posterior part of 

 the body and are too small. Those sponges have very often 

 uneven edges and generally they fit much worse than the cases of 

 young individuals. I believe that the .comparatively small cases 

 result from the difficulty of finding sufficiently large sponges 

 which may be torn off without being damaged. Younger crabs 

 can use sponges which are smaller and therefore younger and 

 more plastic. Perhaps it accounts for the fact that only the 

 cases of small Dromia are usually admirably fitted. 



PUTTING OF THE SPONGE ON THE BACK. 



After having manufactured a case Dromia puts it on the back. 

 This process proved to be an extremely various one, which 

 depends largely on the random position of the sponge. 



i. If we take the sponge off from the crab's back and put it 

 in the aquarium with its concavity directed upward, the animal 

 usually returns to it very soon. Dromia grasps the sponge from 

 both sides with the fore legs (Fig. 2 A] and falls on its back, 

 pulling the sponge over it. The sponge, supported by the legs, 

 lies now with its concavity directed downward (Fig. zB). Lying 

 still in this position the animal begins to turn and to move its 

 case in a horizontal plane until the fore edge of it, in the normal 

 position of the animal directed towards the head, points to the 

 abdomen. An ordinary case has an elliptical circumference and 

 its longer axis lies at right angles to the sagittal axis of the body ; 

 the hind edge is marked by its greater curvature, while the fore 

 edge is somewhat flattened. 



