64 HENRY BURY. 
of satisfactory sections of stage B prevents me from deciding ; 
but the fact that the polian vesicle certainly hangs into the left 
cavity in stage A (figs. 10 and 138), as well as the appearance 
of certain specimens in which the fusion of the two cavities 
does not seem to take place, make it quite possible that this 
mesentery arises very nearly along the line of fusion, which, 
owing to the growth of the curved part of the intestine, has 
assumed rather a peculiar course. 
This, however, is only conjecture; and I must admit that, 
except in the region of the esophagus, I am quite unable to 
determine the exact limits of the body-cavities of the larva in 
the adult Synapta. It is, however, obvious that here, as in 
Cucumaria (16 and 17), it is by no means the simple matter 
which Semon represents it to be. 
The descending part of the intestinal mesentery (lying in 
interradius D, close to radius III) is not continued, in the 
early pupa, on to that part of the intestine which lies posterior 
to the stomach—this part of the intestine being at this stage 
completely surrounded by the ccelom, though whether by the 
right and left cavities combined, or by the former alone (as 
certain early sections suggest) I cannot with accuracy deter- 
mine. In later pupe the intestine is tied to the body-wall by 
nnmerous threads of protoplasm; and only in the young 
Synapte which have completely lost the ciliated rings does the 
fully-developed mesentery, continuing the above-mentioned 
descending mesentery, appear. 
B. ASTERIDS. 
Two very distinct types of development occur among 
Asterids, without, so far as I know, any connecting links 
between them. The most marked difference lies in the be- 
haviour of the larval cesophagus, which survives in the adult 
in one form, but is replaced by an entirely new one in the 
other; there are, however, other important points of dissimi- 
larity as well. 
I propose to take first the type in which a new cesophagus 
is formed, since it has hitherto been but little investigated ; 
