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Journal of Experimental Biology partnership with Dryad

Osmoregulation and Ionic Regulation in Hydra


1. Isolated tentacles of Hydra viridis were placed in solutions of sucrose of different known concentrations. In concentrations of sucrose greater than 0.04 M the tentacles shrank, and it was found that the ectoderm, endoderm, and enteron all contributed to this shrinkage. Both ectoderm and endoderm are highly permeable to water. Calculations suggest that the ectoderm cells are at least as permeable as various other cells which have been investigated by many workers.

2. Shrinkage of the tentacle was very slight in 0.05 M sucrose, and no shrinkage occurred in 0.04 M sucrose. It is therefore suggested that the internal osmotic pressure is equivalent to that of 0.04-0.05 M sucrose.

3. The uptake of the radioactive ions 24Na, 82Br, and 42K by Pelmatohydra oligactis has been investigated. All these ions are concentrated above the levels in the external medium.

4. The relations between internal and external concentrations have been determined for sodium and bromide. The internal sodium concentration is maintained at a steady level with external concentrations varying from 5.4 m.moles/l. to approximately 0.20 m.mole/l. The internal bromide level has been shown to fall as the external bromide decreases from 2.5 to 0.05 m.mole/l. Higher external bromide concentrations could not be used as the animals were not healthy in solutions containing more than 0.25 g./l. NH4Br. The internal bromide is also shown to be depressed by the addition of chloride to the external solution.

5. The results obtained indicate that the cells of Hydra are highly permeable to water, and also capable of maintaining an internal concentration of certain ions well above that of the external medium. This requires some form of regulation of water content and of ionic content. In the case of the latter it is not at present possible to say whether it is an active uptake of cations or anions.