THE METAMORPHOSIS OF ECHINODERMS. 103 
ventral side of the larva; for though the right and left cavities 
are united, the point of union takes place so far to the right of 
the intestine as to make it almost inconceivable that the right 
cavity should have anything to do with this region. The 
dorsal horn of the left body-cavity fuses with the axial sinus. 
In Bipinnaria the early fusion of the two body-cavities 
makes accurate determination of their limits impossible. The 
dorsal horn of the left cavity behaves as in Brachiolaria. 
In Crinoids, again, the ventral horn of the left cavity cannot 
be traced with certainty up to the oral mesentery; but the 
opposite side of this mesentery is unquestionably bounded by 
the dorsal horn. 
These facts, taken together, seem to me to establish almost 
beyond doubt that the growth of the left cavity round the 
cesophagus is complete (except for the possible intervention of 
a mesentery between the two horns), and that the right cavity 
has no connection with this region. Whether the oral mesentery 
is primary or (as Echinids seem to indicate) secondary is a 
question which need not detain us now. 
That the water-pore did not move with the water-vascular 
ring, but retained its original relation to the longitudinal (now 
transverse) mesentery, the facts of ontogeny abundantly testify. 
That it has since moved slightly either to oral or aboral side of 
this position need not surprise us; but in all larve, even after 
the establishment of pentamerous symmetry, it will be found 
practically at the level of the mesentery, and I do not think 
that even in adults it has ever migrated to any large extent. 
The course of the intestine along the equatorial line marked 
by the mesentery, and the opening of the anus in the inter- 
radius of the water-pore, as shown in fig. 48, are best attested 
by the Crinoids; but the former is well enough seen in 
Echinid Plutei also, while in the same group, after meta- 
morphosis, the intestine still coils right round the disc as far as 
the above-mentioned interradius, though the anus does not 
open there. Fig. 23 shows the intestine for the most part on 
the right (aboral) side of the stomach in Bipinnaria, but the 
mesentery is attached to its edge ; and the anus in this larva 
