STUDIES IN THE EMBRYOLOGY OF EOHINODERMS. 443 
merit, in fact, strongly suggests segmentation, but I cannot 
discuss in the present paper whether the pentamerism of the 
adult Echinoderms arose in this way in the skeleton, or 
whether it first made its appearance in the hydroccel; the 
latter appears to me more probable. It is further evident 
from figs. 2 and 13 that, taking the terminals as marking the 
radii, the mouth, anus, and water-pore are at this stage all in 
the same interradius. It may be asked, Why should the 
terminals rather than the water-vascular pouches be taken to 
mark the radii? The fact is that during metamorphosis the 
hydrocoel undergoes such extraordinary changes of position 
that it is doubtful whether any reliance can be placed on the 
position of its pouches in the larva as indicating any per- 
manent relations to’ the rest of the body ; at any rate, it is 
certain that the water-pore is much more constant in its 
relations to the mouth and anus than to the water-vascular 
pouches. Thus we know that in Asterina (17) the most 
anterior tentacular pouch (in front of the water-pore) is 
eventually embraced by that terminal which seems to be 
morphologically the most posterior, i. e. the one just behind 
the anus in fig. 13. It is possible that this is also the case in 
Bipinnaria, but if so, it is the tentacular pouch alone which 
shifts its position; for in fig. 15 we see that, after the connec- 
tion of the terminals with their respective water-vascular 
pouches, the mouth, anus, praeoral lobe, water-pore, and water- 
tube still lie in one and the same interradius (see also 2 and 1, 
pi. vii, fig. 6). The same figure also indicates what I shall 
prove more fully in a future paper, that the left anterior 
enteroccel becomes the so-called “ Schlauchformiger Kanal” 
of the adult. The Ophiurids are still more remarkable than 
Asterina in the behaviour of their hydroccel ; as already 
pointed out, the whole hydrocoel is pushed forward and round 
the oesophagus in such a way that the tentacular pouch imme- 
diately behind the water-tube (most posterior in fig. 2) unites 
with the most anterior terminal, while the pouch just in front 
of the water-tube unites with the terminal which is nearest 
the anus. In this case it is clear that to distinguish the ante- 
