1. The uptake of silver ions by Artemia has been investigated. The staining is localized to the first ten pairs of branchiae. There is no staining of the eleventh pair or of any other part of the animal. The uptake of silver is due to a purely passive precipitation of AgCl within the thickness of the branchial cuticle.

2. The effects of KMnO4 and methylene-blue solutions have also been studied. Their effect is localized to the epithelium under the cuticle of the first ten pairs of branchiae.

3. It is concluded that all these staining reactions demonstrate that the cuticle over the first ten pairs of branchiae is the only part of the external cuticle that is appreciably permeable.

4. Animals whose branchial epithelium has been damaged by a brief exposure to saturated KMnO4 solution have lost the ability to osmo-regulate. They are closely isotonic with their medium, and the range of external concentration tolerated is much restricted.

5. This isotonicity is not due simply to increased permeability, but is due to specific destruction of the mechanism normally excreting NaCl in hypertonic media.

6. Correlation of the physiological effects of KMnO4 treatment with the sharp localization of damage, and the evidence for localized permeability indicates that the epithelium of the first ten pairs of branchiae is the site of active NaCl excretion in hypertonic media, and probably of active uptake from hypotonic media.

7. The ontogeny of this mechanism is traced. In nauplii the dorsal organ is apparently concerned in NaCl excretion. When the branchiae develop the dorsal organ degenerates.

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