STUDIES IN THE EMBRYOLOGY OP ECH1NODERMS. 427 
MetschnikofF (18, pi. iii, fig. 20), at a later stage, described as 
mesodermic; it is really present in the oldest Auricularim, 
though in these it escaped the notice of MetschnikofF and of 
Semon (28). In fig. 25 I have represented part of a section 
through a larva just entering into the pupa stage, in which the 
opening of the water- tube into the anterior enterocoel is clearly 
seen. This section is also useful as illustrating the various 
parts into which the tube, usually spoken of as the “ stone- 
canal/’ stretching from the water-vascular ring (hydrocoel) to 
the water-pore, is divisible; (1) water-tube (= stone-canal of 
Asterid), (2) anterior enterocoel, and (3) pore-canal; these 
three parts are distinguishable in all Echinoderm larvse, 
though the second has been frequently overlooked, and the 
first and third consequently spoken of as one. 
The remnant of the anterior entei’ocoel is also traceable in 
Cucumaria (fig. 26), though I have not yet followed its develop- 
ment. In the series of sections from which this figure is 
taken the water-tube and pore-canal are cut transversely ; the 
columnar epithelium on one side of the cavity in the section 
figured, is continuous with those of the pore- canal and water- 
tube, which appear respectively in sections above and below 
this. The same continuity of epithelia is seen in Synapta 
(fig. 25) in some Echinid Plutei (fig. 9), and in Asterina 
(17, fig. 72). 
Shortly before the metamorphosis of Auricularia into the 
pupa the hydrocoel sends out five pouches (primary tentacles), 
and almost immediately afterwards six smaller ones (alternat- 
ing with the former), five of which become the five longitu- 
dinal water-vessels, while the sixth becomes the primary 
Polian vesicle. The groAvth of the hydrocoel into a ring 
accompanies metamorphosis. 
There is some difficulty in determining the position of the 
water-tube in relation to the closing point of the water-vas- 
cular ring. Semon (28, p. 196) states that it lies opposite one 
of the smaller pouches (longitudinal water- vessels), and on this 
he bases his determination that these longitudinal vessels are 
interradial, and that the five primary tentacles are radial, and 
