HEART AND SCAPHOGNATHITE RATE CHANGES IN A EURYHALINE CRAB,CARCINUS MAENAS, EXPOSED TO DILUTE ENVIRONMENTAL MEDIUM

When the euryhaline decapod Carcinus 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 that is sensed in neither the total osmotic strength of the medium, nor the 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.