Abstract
When the euryhaline decapod C. maenas is transferred from sea water to a medium more dilute than 75% sea water, the frequency of heart beat increases. The cardioacceleratory response is probably mediated by external rather than internal receptors, and the parameter sensed is neither total osmotic strength of the medium, nor concentration of any single ion. Although it is not yet determined whether the heart responds to a hormonal or nervous signal, it is likely that the control mechanisms are similar to those which mediate the water permeability changes in Carcinus and other euryhaline crabs. In somewhat more than half of the crabs tested, the frequency of scaphognathite beating is markedly reduced when the animal is exposed to dilute external medium.

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