THE DEVELOPMENT OF THE CAPE CEPHALODISCUS. 205 
organisation of Cephalodiscus, and as tlieir relative 
position and extent may have a bearing on the subsequent 
processes of metamorphosis, some details may be added to 
what is already known for other species. 
Only a limited number of larvae were available, and several 
of these were, for various reasons, unsuitable for detailed 
examination of the cavities, some being too contracted or 
distorted, others were somewhat broken up, owing' to the 
difficulty of getting whole sections through the yolk mass. 
One or two series of transverse sections were, however, satis- 
factory, and showed the body cavities clearly. In one of 
these, cut into a series of 140 sections, the sixth from the 
posterior end (PI. 14, fig. 22) showed that the two posterior 
body cavities extended backwards beyond the yolk and archen- 
teron. At the 17th section (PI. 14, fig. 23) the body cavities 
are very large, extending almost completely round the yolk, 
but are -separated from each other dorsally by a mesentery - 
and ventrally by the posterior thickening already mentioned. 
That there is a here a ventral mesentery obscured by the 
pressure of the yolk is shown in other series, and it is evident 
in the next or 24th section (PI. 14, fig. 24). Here the upper 
wall of the archenteron has become broadly attached to the 
ectoderm, and the body cavities are beginning to disappear 
from the dorsal side. A few sections further on, at the 25th, 
the beginning of the second body cavity appears on the right 
side at its dorsp-lateral coiner, and, at the 27th section 
(PI. 14, fig. 25), it is of considerable size. At the 29th section 
(PI. 14, fig. 26) it has extended to the right side of the 
archenteron, and in this section the second body cavity of the 
left side appears at the upper angle formed by the ectoderm 
and the wall of the archenteron. That the point at which 
the second body cavities begin on each side is therefore not 
similar is evident from this, and in other series of sections it 
also shows a variation, as, for instance, in one in which it 
begins quite at the lateral wall of the ectoderm. The 36th 
section (PI. 14, fig. 27) shows further advance, and, at the 
45th (PI. 14, fig. 29), the third body cavity has disappeared 
