THE DEVELOPMENT OP ASTERINA GIBBOSA. 249 



radius of the stone-canal with the stomach is a part of this same 

 to'ginal mesentery, with which, however, is continuous a piece 

 0' the wall between dorsal and ventral horns of the left coelom, 

 these two horns being still separated by this wall near their 

 right sides (aboral surfaces). 



Histological Changes during the Metamorphosis. 



Up to Stage G the histology has little changed from that of 

 the larva before metamorphosis. The most striking alterations 

 are those connected with the destruction of the prseoral lobe. 

 PI. XX, fig. 136, gives a specimen of them. This figure, which 

 is taken from the larva represented in figs. 62 — 69, shovvs that 

 the ectoderm becomes invaginated into pockets, and then these 

 pockets completely closed, so that no breach in the continuity 

 of the skin is made. The invaginated portion is then destroyed 

 by amcebocytes as shown in the figure. The peritoneum lining . 

 the stalk ccelom contracts violently, the cells becoming cylin- 

 drical instead of flattened, and the larval muscles very appa- 

 rent. So far as I can make out, these cells are destroyed by 

 amcebocytes of the ccelom. 



In the larva the whole hydrocoele rudiment is lined by cylin- 

 drical cells (PI. XX, fig. 138); but as metamorphosis proceeds, 

 and the hydrocoele increases in size, the cells are stretched so 

 as to become flattened (PI. XX, fig. 139); they retain their 

 original character only in the rudiments of the tube-feet (PI. 

 XXI, fig. 149) and terminal tentacles. The first trace of the 

 adult nervous system appears in Stage F in the ectoderm 

 covering the watei-vascular ring, — that is, the portion of the 

 hydrocoele between the primary lobes. The ectodermal cells 

 become long and filamentous, with their nuclei set at diff"erent 

 levels, and amongst their bases (PI. XXI, fig. 140) appears a 

 tangle of fine fibrils of excessive tenuity, so that the highest 

 magnification is required to make them out; this is the first 

 trace of the adult nervous system. 



Ludwig talks of cells stretched parallel to the surface under 

 the ectoderm, which he supposed to become the bipolar gan- 

 glion cells of the nerve-cord ; but the cells in question, if I 



