56 HENRY BURY. 
is clear also that the closure of the water-vascular ring is 
effected on the ventral side, just at the base of the polian 
vesicle. How near to the middle line this junction occurs is, 
however, difficult to determine. I have failed to observe the 
polian vesicle in external views, owing, no doubt, to its being 
turned inwards towards the body-cavity ; and in sections so 
much distortion almost inevitably occurs that the exact middle 
line is hard to determine. Moreover it will be noticed (figs. 
4 and 5) that, owing to the asymmetry of the larva in stages 
A and B, the middle line of the stomach does not correspond 
with that of the larva as a whole. The polian vesicle, how- 
ever, is certainly to the right of the middle line in many larve 
at the close of stage A (fig. 13), though there is some reason 
to think that in later stages it moves back again somewhat to 
the left: at avy rate, the closure of the water-vascular ring, 
whether to the right or left of this line, is not far removed 
from it. 
In Cucumaria, Ludwig (17, p. 607) thinks that this closure 
occurs on the right side, not in the interradius of the polian 
vesicle ; and in view of the remarkable discrepancies existing 
among Echinoderm larve on this point, it must be admitted 
that he is possibly right, but until every stage has been traced 
(Ludwig admits that he has missed the actual completion of 
the ring), it would be rash to assert that Cucumaria and 
Synapta differ in this respect. 
With the commencement of stage B and the fornatien of 
the atrial cavity (to be further described later on) the hydro- 
col ring, which is only completed at the commencement of 
this stage, assumes a new position. In stage A its oral sur- 
face was directed nearly towards the ventral surface of the 
larva, and only slightly towards the anterior end; now, how- 
ever, it faces (approximately) the aperture of the atrium, and 
as this moves towards the anterior end, the water-vascular ring 
comes to lie more and more nearly at right angles to the 
longitudinal axis, a position which it finally assumes in the pupa 
or “barrel”? stage. In stage B, however (fig. 5), the aperture 
of the atrium is not yet polar, and the oral surface of the 
