MUSEUM OF COMPARATIVE ZOOLOGY. 21 
out and above, the atrium of the adult. (Plate III. Fig. 25; Plate IV. 
Fig. 32, atr.) Since the lophophore curves rapidly downward to the 
anus and does not extend behind it, the act of cutting off the lower part 
of the atrio-pharyngeal cavity from the upper (atrium proper) does not 
continue behind the anus, which therefore opens directly into a part of 
the atrium. This part has the form of a compressed funnel, and is 
bounded behind and laterally by the kamptoderm, and orally by the 
hinder ends of the lophophoric ridges, and also, since the latter do not 
meet in the median plane, by the pharyngeal cavity. Thus it has come 
about that the anus, which at first opened into the common atrio-pha- 
ryngeal cavity of the bud, has now, in the separation of the two regions, 
come to lie near their point of division posteriorly, but to open distinctly 
into the atrial cavity. The more pronounced separation of the part of 
the atrial cavity into which the anus directly opens from the remainder 
of the atrium takes place much later, and will be described further on. 
In Figure 33, the ring canal (can. cre.) is seen to be already formed. At 
this stage it is found on one side only, the left, if one looks at the poly- 
pide from the tip of the branch. It occurs in only four sections (each 5 a 
thick), being found on the next section behind Figure 33, and on two sec- 
tions nearer the oral end. At its oral extremity, it terminates blindly 
as a thickening of the outer layer of the bud; at its anal end, one sees 
cells of the outer layer extending out partly over the canal, but failing 
to enclose it; in the next section the mesoderm is undisturbed, In sim- 
ilar sections of an older polypide (corresponding in age approximately to 
Plate IV. Fig. 35), the canal is found on both sides, and near to the oral 
end, but at about the middle of the series (cf. Fig. 35) it is found to 
open again into the body cavity. I therefore conclude that the ring 
canal makes its first appearance at the base of the lophophore in a 
region just oral of the middle of the polypide. Exactly how it arises, 
whethor by a growing together of the lips of a shallow furrow formed 
from the mesodermal layer, or by the formation of a pocket, which, elon- 
gating, penetrates between the inner and outer layers of the polypide at 
the base of the nascent lophophore, I have not been able to determine. 
Two facts induce me to believe that the later formation of the canal 
oralwards results from the penetration of a sac-like mass of mesodermal 
cells between the two layers of the polypide at the base of the nascent 
lophophore. One usually finds, (1) as in Figure 33, can. crc., a double 
mesodermal wall between the lumen of the canal and the cœlom, and 
one layer between the former and the inner layer of the bud; and 
(2) at the oral blind end of the ring canal a number of loose cells 
