432 
H. BURT. 
not yet certain whether in it the hydrocoel ever forms a horse- 
shoe curve shut off from the anterior enterocoel, as it does in 
Asterina, or whether the new oesophagus simply grows through 
it and perforates it as Metschnikoff asserts (18 and 19 ). 
All this variation is certainly very puzzling ; but if, as seems 
to me necessary, we regard the interradius of the water-tube 
as a fixed point, we are almost bound to conclude that the 
present position of closure of the water-vascular ring is secon- 
dary, at any rate in most groups. We have already seen 
reason to doubt whether the derivation of the hydrocoel from 
the anterior enterocoel, which obtains in the ontogeny of most 
groups, is really phylogenetic, and we are now tempted to 
ask whether the whole development of the hydrocoel, up to the 
time when it forms a complete ring round the oesophagus (the 
earliest stage in which all Echinoderms agree), has not under- 
gone secondary changes which completely mask its true phylo- 
genetic history. It is to be hoped that further investigations 
may throw some more light on this point, which at present 
forms one of the most insoluble, as well as the most important, 
questions in Echinoderm morphology. 
III. The Skeleton. 
The greater part of the development of the skeleton belongs 
to the pentagonal stage, and with this we are not at present 
concerned ; nor need we mention the purely larval skeletons of 
Ophiurid andEchinid Plutei — only such parts of the per- 
manent skeleton as are developed in the Dipleurula will be 
considered here. 
Ophiurids. — No satisfactory observations have hitherto 
been made on the first appearance of the skeleton in this 
group. Ludwig ( 16 ), in his valuable studies on the skeleton 
of Amphiura, found that the radials and terminals were present 
before any other plates of the aboral surface, but he was unable 
to determine which of these sets was the first to appear. 
Fewkes ( 10 ), working on the same animal, states positively (p. 
139) that the radials appear before the terminals, though he 
