THE density of the living Ctenophore Pleurobrachia pileus (O. F. M. ) has been determined recently at Plymouth by two distinct methods. In the first case ten specimens all having the approximate diameter of 1 cm. were introduced into a 5 ml. density bottle, together with sea water, and weighed. The volume of the animals was calculated by measuring their diameter carefully and assuming that they were spheres. The density was found to be 1.02742 at 13.0° C. In the second case more than a hundred specimens were introduced into a density bottle and weighed, but the volume was found by the more accurate displacement method1. Here again the density was found to be 1.02741 at 13.0° C. The water content was also estimated and found to be 94.73 percent.