The Development of Echinus esculentus. 



271 



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. 



d 



M 



pq 



