THE METAMORPHOSIS OF ECHINODERMS. 63 
approach one another in fig. 8. From this point the line of 
fusion runs obliquely forward, as we have seen, along the 
posterior edge of the “ventral horn” of the left body-cavity. 
Posteriorly the fusion usually (but perhaps not invariably) 
extends to the anterior margin of the intestine, apparently 
nearly in the middle line, but of this I cannot be quite sure. 
Over the transverse portion of the intestine the left body- 
cavity (which at this level is much smaller than the right) 
pushes further across to the middle line (fig. 16) than the 
right body-cavity, and then passes even on to the right side, 
ending altogether just before the level of the posterior end of 
the stomach is reached. Its margin thus takes an oblique 
course, from left to right; but as the margin of the right 
body-cavity is still far removed from it at this stage, we cannot 
speak of a definite mesentery at this point. The right body- 
cavity extends somewhat further posteriorly than the left ; but 
very little behind the posterior end of the stomach it also 
comes to an end. 
Of stage B I have obtained but few examples, and none of 
them, I regret to say, show the mesenteries satisfactorily, 
In the fully-formed pupa the dorsal mesentery remains very 
much as we left it at the close of stage A, except that, as 
already mentioned, it is now continuous with the mesentery of 
the water-tube. Posteriorly, at about the level of the junction 
of the intestine and stomach (but in this stage a long way 
from the posterior end of the latter), it passes sharply round 
to the ventral side—in other words (since the original dorsal 
and ventral surfaces are no longer evident), it passes from the 
middle of interradius A into interradius B, thus forming the 
short transverse mesentery above mentioned. From this point 
it is continued into a mesentery which runs along the left edge 
of the ascending portion of the intestine, curves round the 
anterior margin of the latter, external to the polian vesicle 
(fig. 15, ‘‘ mesentery”), and then descends for a short distance 
along the right edge of the descending portion of the intestine. 
How far this intestinal mesentery marks the division of the 
two body-cavities, and how far it is a new growth, the absence 
