Abstract
1. In Chaetopterus gradually adapted to lowered salinities, coelomic fluid osmolarity and sodium and chloride concentrations conform to ambient seawater at salinities from 1100 to 630 mOsm.2. Experimental animals were unable to tolerate salinities below 630 mOsm for extended periods of time.3. Worms transferred directly to hypoosmotic stress solutions down to 681.8 mOsm gained weight (volume) rapidly, reaching a maximum in approximately 1.5 hours. At this point, the coelomic fluid was isoosmotic with the external medium. After 1.5 hours, there followed a decrease in volume, which as shown in one experiment, culminated in a return to initial volume values.4. Transfers back to full-strength seawater indicate that salt efflux as well as water influx occurred.5. It is concluded that Chaetopterus is a volume-regulating osmoconformer over its viable range of salinities.6. Current theories of the mechanism of isoosmotic volume-regulation are discussed and a mechanically linked volumestat model is proposed.7. T...
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