ABSTRACT
As is well known, the hydrocœle or water vascular system of Echinoderms arises from a portion of the left cœlom. The early larvae being bilaterally symmetrical, a great deal of discussion has taken place as to whether any definite portion of the right cœlomic cavity corresponds with the hydrocœle, and if so, whether this portion gives rise to any adult structure which can be identified. There has also been much speculation regarding the bilateral ancestor. On both of these problems a study of double hydrocœle—that is, the condition in which a right as well as a left hydrocœle appears—is calculated to throw light. While this is true for any kind of Echinoderm, it applies particularly in the case of A. rubens, since there is evidence that the ontogenetic history of our species belongs to the least modified type of larval development we yet know of within the phylum (4, p. 279). Again, various points in normal development are difficult to determine with certainty, on account of the great and rapid change of symmetry which occurs at metamorphosis. Welcome additional light on a. number of these points (p. 72) has accrued from a study of double hydrocœle.
