1. Reasons are given for believing that the uptake of Na+, Cl-, and NaCl by the crayfish occurs through the gills.
2. A crayfish in fresh water, with a Cl concentration of about 0.2 mEq./l., can) by active Cl absorption, compensate entirely for Cl lost in the urine.
3. The carbonic anhydrase activity of the gills is markedly higher than that of other tissues of the crayfish, but the equivalent CO2 output of the crayfish is far in excess of the equivalent Cl absorption per unit time and weight and thus fails to warrant the supposition that Cl absorption is of respiratory importance.
4. The carbonic anhydrase activity of the soft integument of the lobster, before and after molting, and of the hypodermis of the hard-cuticled animal is almost identical and of the same order as that of other tissues of the lobster.
5. The concentration of the electrolytes was about 7.5 mEq./l.; i.e., considerably lower than in the blood of the crayfish. Cl- can be taken up independently of the complementary cation. Na+ can be taken up independently of the complementary anion. K+ and SO4= are not taken up at all. In pure NaCl, the Na+ and Cl- are absorbed evidently largely together. Ca++ is absorbed only in newly molted animals and in animals preparing to molt but is not absorbed by hard-cuticled animals not preparing to molt. Ca++ is taken up independently of Cl- in pure CaCl2.
6. Newly molted animals absorb Ca++ at a rate exceeding that of the absorption of other absorbable ions (Na+ and Cl-) in the same equivalent concentration.
7. A crayfish utilizes the Ca++ in fresh water in the calcification of its cuticle. Since the animal does not swallow water, the Ca++ must enter through the exterior. Reasons are given for believing that, unlike Na+ and Cl-, Ca++ is absorbed directly from the exterior by the integument and does not enter the body through the gills.
8. During molting, only about 4 per cent of the raw ash and 2.3 per cent of the organic material of the old cuticle is resorbed.