STUDIES IN THE EMBRYOLOGY OP ECHINODERMS. 431 
The water-tube is formed after tbe hydrocoel has become 
lobed. 
These facts seem to show that even if the derivation of the 
hydrocoel from the anterior enterocoel is of phylogenetic sig- 
nificance, there must have been a subsequent time when the 
two cavities were entirely separated, otherwise it is difficult to 
understand why the primary communication does not always 
coincide with the water-tube, as it does in Holothurians and 
Echinids, but is sometimes so far distinct from it as to be in a 
different interradius (Asterids). It seems more rational to 
regard the condition found in the two former groups as secon- 
dary (perhaps as a physiological hastening of the connection 
between the hydrocoel and the exterior), and to suppose that 
the water-tube is a secondary structure belonging to a compara- 
tively late stage in the phylogeny of Echinoderms. 
(3) Closure of Water-vascular Ring. — Ludwig (14, 
p. 45) has already alluded to the variation in the point of 
closure of the water-vascular ring with regard to the position 
of the water-tube (stone-canal) ; but as I have been able to 
collect some data which he did not possess, it will be well to 
review the whole matter with some care, and for this purpose 
it seems to me that the diagram (fig. 28) will be of more use 
than the most detailed description of the facts. For reasons 
which will be given later the interradius of the water-tube is 
placed anteriorly ; the anterior part of the water-vascular ring 
lies on the dorsal side of the oesophagus, while the posterior 
part lies beneath it. The positions marked for the closure of 
the water- vascular ring rest principally on my own observations, 
but that of Asterina is given on Ludwig’s authority, while that 
of Ophiurids has been already described by Metschnikoff, whose 
account I can confirm. Barrois’s account for Autedon differs 
from mine in that he puts the point of closure on the other 
side of the water-tube, though in the same interradius (5, 
p. 608). The case of Holothurians cannot be settled till we 
know whether the radii are marked by the primary tentacles 
(Holoth., I), or by the longitudinal vessels (Holoth., II). 
Bipinnaria has not been very satisfactorily studied, and it is 
