204 
J. D. F. GILCHRIST. 
This lumen appears somewhat triangular in longitudinal 
section ; in sagittal sections of some advanced embryos it is 
seen that the part of the yolk forming the upper portion or 
roof of the central cavity has disappeared, and it is now in 
connection dorsally with the archenteron. The consumption 
of these yolk granules in this particular region is apparently 
associated with the active growth in the tissues immediately 
adjacent to it. More posteriorly, a part of the roof of the 
lumen is still present, and a transverse section of this part 
would show a circular cavity in the yolk. This is very well 
illustrated in Andersson’s figs. 73-78, and in transverse 
sections of younger larvae of this species. He still regards 
this diminishing cavity as the archenteron, and its walls, 
including the homogeneous substance which is interpreted 
here as the detritus of yolk granules attacked by vitellophags, 
as the endoderm formed by invagination. The very large 
cavity in front of and above the yolk can in this case only be 
considered, as he does, to represent the cavity of the pro- 
boscis. Later embryos, however, show that the roof of the 
yolk lumen disappears even from the posterior part, just as it 
did in the anterior, so that no yolk cavity is left at all. This 
is seen in transverse sections (PL 14, figs. 23-30) of an 
advanced larva the exact age of which cannot be determined; 
it was found crawling over a ccenoecium and kept alive for 
about two days afterwards. Here the roof of the yolk lumen 
has entirely disappeared, though the floor, or ventral part, 
still persists as a fairly large mass, with the cavity of the 
archenteron above it. The ventral part of the yolk is 
probably used up in the next step in the metamorphosis of 
the larva, as yet unknown, but probably most marked on the 
postero-ventral aspect of the larva, where this yolk mass 
lies. 
(2) The body cavities of the larva. — The four 
posterior body cavities can usually be seen distinctly in suit- 
ably prepared material, provided there is not too great con- 
traction of the tissues. Their epithelial lining can also be 
sufficiently distinguished. As they are of importance in the 
