100 HENRY BURY. 
occurred, for it is quite possible that the result was produced 
by a complicated system of hypertrophies and atrophies ; but 
the predication of an actual migration gives us the simplest 
process for descriptive purposes, and at the same time empha- 
sises my objection to MacBride’s assumption that hypertrophy 
of the left side has been the cause of the change of symmetry. 
Though the water-vascular ring almost necessarily moved 
with the cesophagus, which it surrounds, the water-pore, lying 
over the anterior end of the stomach, was not involved in this 
movement. The moment the cesophagus began to move it 
came in contact with the margin of the left body-cavity, which 
seems to have become invaginated to receive it, and by the 
time the cesophagus reached the centre of the left side of the 
stomach, to have surrounded it completely (see figs. 47 and 48), 
possibly forming a mesentery to support the water-tube—but 
to that we shall return. Figs. 46 and 47 will render the tran- 
sitional stages sufficiently intelligible, while figs. 48 and 49 
show the arrangement of the principal organs when the ceso- 
phagus has reached its resting-place in the centre of what was 
the left side of the stomach. The water-tube is elongated so 
as to run over the surface of the stomach from the water- 
vascular ring to the water-pore, and parallel to it as before 
runs the anterior enteroccel (ampulla). Almost at the same 
level as the water-pore the originally longitudinal mesentery 
forms a sort of equatorial band round the stomach, and in this 
plane lies the intestine, which has not changed its position 
relative to the stomach and mesentery. 
It will be noticed that in these diagrams (figs. 48 and 49) I 
have assigned positions to the tentacles which do not quite 
accord with the supposition of a simple movement of the 
hydroceel into the left side. In the bilateral form the tentacles 
were arranged symmetrically about a dorso-ventral plane, one 
on the ventral side being median. If the movement into the 
left side were as simple as I have so far represented it, this 
plane of symmetry would now lie approximately along the line 
X-Y in fig. 48 (its exact position depends on how we construct 
fig. 44). But this, if we assume the water-tube still to open 
