larva is 11 days old, so that by that time the coelom of the larva has completed its transverse segmentation (fig. 4, d).
About 13 days a hollow pouch-like invagination of the ectoderm takes place in the region where the hydroccele is situated, and this is the first trace of the ectoderm of the oral disc of the Echinus (fig. 2). At the same time four horizon- tally placed festoons of the longitudinal ciliated band become separated from the rest and form the "ciliated epaulettes," which are the main organs of locomotion of the larva in the later stages of its development (see fig. 3).
At 16 17 days an outgrowth takes place from the hinder end of the right anterior ccelom. It is directed dorsally, and its free end thickens into a nodule of cells which for a brief period is connected with the rest with a string of cells which soon breaks. The nodule then becomes hollowed out, and forms a perfectly closed vesicle lying beside the " ampulla " of the stone-canal. It is the rudiment of the "madreporic vesicle" or "espace sous-madreporique " of the adult, and represents, as I have shown in the case of Asterina gibbosa, a rudimentary fellow of the hydrocoele. We may interpret the transient cord of cells as a right stone- canal (see figs. 2 and 3).
Keviewing the development thus far described, it appears that the coelom of the right side of the larva undergoes similar changes to those experienced by the ccelom of the left side, but they are
slower in their development and this agrees with the facts observed in the case of Asterina gibbosu.
The important fact that the coelom of the Pluteus underwent trans- verse segmentation was first made known by Bury* in 1889. His observations were, however, confined to specimens picked out of collections of Plankton, and the series of developmental stages which he obtained was not without considerable gaps. On this account the
- " Studies in the Embryology of Echinoderms," ' Quart. Jour. Micro. Sci.,'
Tol. 29, 1889.
