220 WILSON. [VoL. II. 
section on the right is through a mesentery of the first order, on 
the left through one of the second order. The line 3 marks the 
position of the free edges of the tertiary mesenteries. The 
latter are not provided with filaments. On each side of the 
section, the mesentery is divided into a central and peripheral 
part (R.P.) by the calcareous theca, 7%. (When the animal is 
expanded, the peristome is lifted high above the level of the 
skeleton. It then embraces the whole width of the animal, 
the tentacles forming a dense ring round its edge, while the 
mouth is narrowed to a slit-like opening.) 
On the right side of Fig. 50 the epithelium of the cesophagus, 
cz, is directly continuous with the filament 1, the mesentery 
being complete. On the left side the mesentery being incom- 
plete, the cesophagus has a free edge. Now if my view is cor- 
rect, not only the lining epithelium of the cesophagus is ecto- 
dermal, but the epithelium x, which forms the gastric covering 
of the cesophagus and peristome between the complete mesen- 
teries, is likewise ectodermal: the ectoderm here, as in the 
larva, is reflected round the lip of the cesophagus, and extends 
upwards until it reaches the secondary mesentery, down which 
it courses as the filaments. Fig. 55 is a more highly magnified 
view of the lower part of the cesophagus, as shown in the left 
half of Fig. 50. The lining epithelium of this part of the 
cesophagus, the Jower third, is composed of slender supporting 
cells. The upper two thirds contain large nettle cells, and in 
the region of the mouth mucus cells. The epithelium on the 
outer surface of the cesophagus, is for some-distance (compare 
Fig. 55 with Fig. 50) exactly like the lining epithelium, except 
that it contains a few yellow cells. Then comes a region of 
vacuolated cells, which is followed by a very low epithelium made 
up of exceedingly small cells, the exact shape of which I could 
not determine. The low epithelium is continued up the cesopha- 
gus and over the peristome, and is continuous with the filaments 
on the secondary mesenteries. These filaments, though of large 
size at a lower level, gradually become very small as they ap- 
proach their upper limit, and by this means run without any 
break into the low epithelium covering the peristome. The fila- 
ments on the third pair of mesenteries in the larva were like- 
wise very small in the upper part of their course, though, to be 
sure, in my oldest larval stage they as yet only existed in this 
part. 
