48 HENRY. BURY. 
and about equal in size; but now a small finger-like process 
is pushed out from the anterior end of the left cavity (fig. 1) 
which very soon meets the posterior end of the hydroceel, and 
grows forward along its ventral surface. This process, which 
has no fellow on the right side, was recognised by Metschni- 
koff (21, pl. ii, fig. 11), but seems to have escaped Semon’s notice. 
While it is forming, the two body-cavities begin to -grow 
towards one another on the ventral side, a little behind the 
cesophagus, the growth both of this part and of the anterior 
process of the left cavity being marked by great pseudopodic 
activity of the cells forming the walls of the cavities (fig. 1). 
About twenty-four hours after the appearance of this pro- 
cess of the left body-cavity, we reach the stage represented 
‘in fig.2. The body- cavities have come close together ventrally, 
but not symmetrically, a portion of the left cavity overlapping 
the anterior edge of the right (fig. 3). It is further to be 
noticed that while the left cavity now extends anteriorly all 
along the ventral face of the hydrocel (“ oral cavity,” fig. 3) 
it scarcely extends so far posteriorly as the right cavity. In 
sections (with the help of which I have confirmed all the 
observations embodied in the figures) the external wall of the 
left body-cavity exhibits a marked thickening in the region in 
which the two cavities approach one another; occasionally, 
though rarely, there seems to be already a fusion of the two 
cavities at this point. 
The Auricularia has now reached its latest stage of develop- 
ment, and in a very few hours the whole aspect of the larva is 
changed, and the metamorphosis from the bilateral into the 
radial form has begun. 
As several points in this change have been overlooked both 
by Metschnikoff (21) and by Semon (82), we must follow it 
in considerable detail. The onset of metamorphosis is marked 
externally by the breaking-up of the ciliated bands, the col- 
lapse of the stomach, and the growth of the hydrocel round 
the cesophagus; and as far as possible these various groups of 
phenomena will be dealt with separately, though it is impos- 
sible to keep them wholly apart. 
