1. The respiratory movements of the barnacle Balanus and of the phyllopod Cheirocephalus are not accelerated either by decreased oxygen or by increased carbon dioxide tension in the water.

2. The rate of movement of the scaphognathite of the crayfish Astacus is accelerated by a fall in oxygen but not by a rise in carbon dioxide tension of the environment.

3. The rate of movement of the pleopods of the fresh-water isopod Asellus is accelerated by a fall in oxygen but not by a rise of carbon dioxide tension.

4. In the sea-shore isopod Ligia, submerged in sea water, the rate of pleopod movement is not accelerated by a decrease in oxygen tension below that in equilibrium with air, but a rise in oxygen tension above the latter value slows respiratory movements. Carbon dioxide has no accelerating effect.

5. Both a decrease in oxygen and an increase in carbon dioxide tension accelerate the respiratory pleopod movements of the amphipods Gammarus pulex and G. locusta.

6. Whereas in the fresh-water G. pulex the quickened respiratory rhythm is permanent at each decreased oxygen and increased carbon dioxide tension, in the marine G. locusta these changes in rate of beat are transitory. This is correlated with the greater oxygen consumption of the marine species.

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