Abstract
Abstract 1. The barnacle Balanus improvisus succeeds in fresh water as well as in sea water. The euryhalinity depends partly on hyperosmotic regulation of the haemolymph and partly on cell volume regulation. 2. The animals osmoconform in waters above 500 mOsm and osmoregulate in more dilute sea waters, showing strict homoiosmoticity below 100 mOsm. Haemolymph and maxillary gland fluid are isosmotic and have equal chloride concentrations. 3. Seventeen free amino acids are found in thorax muscle tissue in amounts varying with haemolymph osmolality. Proline reaches unique values of 0.7 M at hypersaline sea water. 4. The relative water content of muscle tissue varies little with changes in haemolymph osmolality pointing to a regulation of cell volume. The adjustments of the intracellular amino acids, especially proline, assist in this regulation. 5. The intermoult cycle does not significantly influence the measured parameters.
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