Abstract
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 quantity 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 ...
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