102 HENRY BURY. 
with relation to the longitudinal mesentery and water-tube, 
which justifies us in asserting that here, too, there is an actual 
migration over the left side of the stomach. 
All these differences between the various larve are easily 
intelligible as shortenings in ontogeny of the phylogenetic 
migration—the shortest process of all being foundin Bipinnaria 
asterigera, in which both hydroccel ring and permanent ceso- 
phagus are produced in the positions which they will ultimately 
occupy. 
In close connection with this lies the point already alluded 
to in connection with the bilateral ancestor—the angle which 
the plane of the water-vascular ring forms with that of the 
mesentery. If the facts above given are referred to, it will be 
found that here too we have represented the steps by which the 
ontogenetic process has been shortened—the two planes being 
at right angles to one another in Ophiurids; inclined at a 
lesser angle in Brachiolaria and Crinoids; and parallel from 
the first in the Bipinnaria. 
The facts just quoted with regard to the migration of the 
cesophagus and water-vascular ring greatly strengthen the con- 
clusion that the Ophiurid position of the latter—at right angles 
to the mesentery—is the primitive one. 
The encircling of the cesophagus by the left body-cavity has 
been sufficiently emphasised in the early part of this paper; 
but the frequent occurrence of a communication between the 
right and left cavities makes it often difficult to say how far 
this process is carried, and how far we are justified in asserting 
that the mesentery of the water-tube (‘ oral mesentery,” let 
us call it for brevity’s sake) is really bounded on both sides by 
the left body-cavity. The following summary of the facts will 
help us to a conclusion. 
In Echinids there seems to be no fusion of the body-cavities 
before metamorphosis ; but the dorsal and ventral horns of the 
left cavity fuse completely, the oral mesentery being formed 
later. Further investigation of these larve is, however, desirable, 
In Brachiolaria it is fairly certain that the left body-cavity 
bounds the oral mesentery (containing the axial sinus) on the 
