By virtue of two properties, lysis on reaching a critical volume, and a discoidal shape, the hemerythrin-containing corpuscles of the coelomic fluid of Phascolosoma gouldi have proven to be excellent material for the quantitative estimation of permeability constants. The constants evaluated by the method of Jacobs from times of lysis show a general similarity between these cells and unfertilized Arbacia eggs in respect to permeability to water and a series of non-electrolytes. The Phascolosoma cells also resemble Arbacia eggs in their low permeability to both cations and anions.