THE DEVELOPMENT OF ASTERINA GIBBOSA. 367 
radius of the stone-canal with the stomach is a part of this same 
original mesentery, with which, however, is continuous a piece 
of the wall between dorsal and ventral horns of the left ccoelom, 
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 preoral lobe. 
Pl. 27, fig. 186, gives a specimen of them. This figure, which 
is taken from the larva represented in figs. 62—69, shows 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 ameebocytes as shown in the figure. The peritoneum lining 
the stalk ceelom 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 hydroceele rudiment is lined by cylin- 
drical cells (P1. 27, fig. 188); but as metamorphosis proceeds, 
and the hydroceele increases in size, the cells are stretched so 
as to become flattened (Pl. 27, fig. 189); they retain their 
original character only in the rudiments of the tube-feet (PI. 
28, fig. 149) and terminal tentacles. The first trace of the 
adult nervous system appears in Stage F in the ectoderm 
covering the water-vascular ring,—that is, the portion of the 
hydroceele between the primary lobes. The ectodermal cells 
become long and filamentous, with their nuclei set at different 
levels, and amongst their bases (Pl. 28, 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 
