METAMORPHOSIS OF ASTERINA GIBBOSA. 231 



2. Hydro-Enterocœl. 



In treating of this system I start witli stage B/C, figs. 1 & 2, 

 PI. XVIII. As may be gathered from these figures there are, in 

 this stage, only two cavities entirely separated from each other, 

 one being the loft posterior enterocœl and the other including the 

 hydrocœl, the anterior enterocœl and the right posterior enterocœl, 

 all these latter freely communicating with each other (fig. 6). 

 There is no natural boundary betw^een the anterior and the right 

 posterior enterocœl ; but I conceive it to be determined by a 

 transverse plane passing through the pore-canal, as I have done 

 in the case of Ästerias. We also see that the pore- canal lies at 

 the posterior extremity of the anterior enterocœl, on the left 

 side. As implied in the statement made above, the left and the 

 right posterior enterocœl are entirely separated from each other, 

 there being a mesentery on the dorsal as well as on the ventral 

 side (fig. ü). In stage C/D this separation becomes incomplete, 

 the mesentery disappearing for a short extent (between lines a 

 & b, fig. 4; fig. 7 h) on the dorsal side, and on the ventral side 

 as far posteriorly as line c (fig. 4). At this stage, therefore, 

 all the cavities communicate with each other directly or in- 

 directlv. In stao;e D the communication on the dorsal side still 

 exists, but the two cavities are again completely separated from 

 each other in the next stage (E, fig. 11) ; while the mesentery 

 on the ventral side gradually disappears. This process of dis- 

 appearance of the primary ventral mesentery begins in stage C 

 and is accompanied by the formation of another mesentery on 

 the right side of the body. This secondary mesentery always runs 

 obliquely to the long axis of the larva, as has been correctly 

 figured by Ludwig ['82, figs. 31, 32], and is formed by the 



