174 



J. F. ABBOTT. 



TABLE II. 



SUMMARY. 



1. The fiddler crabs of the genus Uca are able to endure 

 abrupt and profound changes in the osmotic density of the 

 water which they enter, passing from sea water into ordinary 

 "fresh" water with impunity. 



2. They are able to spend long periods out of the water by 

 the aid of a mechanism by means of which they store up a quan- 

 tity of water above the gills and aerate it directly by contact 

 with the air. 



3. Their gill membranes are only relatively not absolutely 

 impermeable. In the presence of pure distilled water they both 

 lose salts (as indicated by the titratable chlorine) and also gain 

 weight through the absorption of water. 



4. They will live a much longer time in small quantities of 

 distilled water than in larger quantities. This is probably due to 

 the fact that the salts diffused out from the tissues although 

 small in amount are sufficient, in a small quantity of water, to 

 raise the osmotic tension of the latter sufficiently to inhibit its 

 solvent action on the gill membranes and thus to retard the 

 further diffusion of salts. In a larger quantity of water this 

 point cannot be reached before the debilitating effect of the loss 

 of salts results in the death of the crabs. 



