THE METAMORPHOSIS OF ECHINODERMS. 85 
If this is admitted, there cannot, I think, remain much 
doubt that the left body-cavity, in the course of its movement, 
encircles the wsophagus; though whether it ever forms a 
mesentery in the interradius of the water-tube, must be left 
for future investigation to decide. 
Another point requiring further evidence is the behaviour of 
the intestine. If the movement of the mesentery be admitted, 
it is scarcely possible to doubt that the stomach moves with 
it; and it would be at least a plausible conjecture that the 
intestine, if it survived, would retain its primitive relation to 
the stomach and body-cavities, and so assume a position trans- 
verse to the axis of the adult esophagus. As a matter of fact 
I believe in most larve the intestine and anus disappear almost 
the moment metamorphosis sets in; but in a few larve I 
have thought that I could trace the intestine, at the stage shown 
in fig. 42, bending over to the right, exactly as we should 
expect it todo. I have not, however, succeeded in obtaining 
sections which prove this; and until I have done so, I do 
not like to assert that it really occurs, though analogy renders 
it extremely probable. 
Up to the stage immediately preceding metamorphosis (fig. 
40) the two body-cavities are practically equal in size; and it 
would seem from fig. 41 that no great difference between them 
exists even when the radial symmetry is fully acquired. 
At this stage (5, fig. 6) the water-pore is nearly at the edge 
of the disc; and from close by it starts (much later) the genital 
rachis, which grows as a ring round the body (18, p. 138). 
Subsequently, when the arms grow out, the water-pore moves 
on to the ventral surface, while the radial portions of the 
genital rachis remain dorsal to the arms, so that the whole 
rachis assumes the form of an undulating cord, described by 
Ludwig as an aboral vascular ring. Now I have no evidence 
to offer except analogy; but I would suggest as possible, and 
worthy of investigation, that this genital rachis may mark, as 
it does in Crinoids, and I believe in Asterids and Echinids also, 
the position of the larval mesentery—the original line of 
separation between the two body-cavities, In this connection 
