No. 2.] LARVA OF CTENOPHORA ANGUSTIPENNIS. 547 
posed of a single layer of columnar cells and is thrown into 
numerous, deep, irregular, longitudinal folds, which at times 
cause the lumen to be almost closed. The inner margin of the 
epithelial layer, unlike that of the epithelium of the ceeca, forms 
a straight line. This layer secretes the heavy, inner chitinous 
layer, depositing the chitin in irregular laminze. In the anterior 
portion of the tube the lumen is nearly filled by the long, 
stiff hairs which project into the cavity. (Fig. 14.) These 
hairs are entirely lacking in the posterior portion, which is 
also distinguished by having no signs of the longitudinal 
muscle strands. 
The proventriculus comprises the adjoining abruptly dilated 
portion. At the point of juncture the cesophagus is sur- 
rounded by the fine sphincter muscle, so that the entrance to 
the cesophagus may be completely closed and so keep food 
from passing back up the cesophagus. (Fig. 15.) There is 
also a large cesophageal invagination. This has the same cell 
structure as the posterior portion of the cesophagus, and 
extends for about the length of the proventriculus, when the 
line separating the ventriculus and the proventriculus is taken 
to be the beginning of the ventricular ceca. The wall of the 
proventriculus is composed of a single layer of secreting 
epithelium and is surrounded by a thin muscular membrane. 
This membrane is composed of a layer of circular muscles and 
an incomplete layer of longitudinal muscle fibers. The epithe- 
lial layer shows no signs of great activity as the inner cell 
margin is straight and no discharged globules are present. 
There is no sharp line of division between the proventriculus 
and the vertriculus. The ventriculus proper bears at its 
anterior end four elongated pouches, the ventricular czeca. 
These are not alike in structure, two of them being nearly twice 
as long as the other two. They differ in this respect from 
the gastric ceeca of Holorusia, where all four are of the same 
size. The longer czeca extend along the ventriculus for about 
one-fifth of its entire length. The transition of the epithelium 
of the ventriculus to that of the caecum is very sudden. 
(Fig. 16.) The epithelium exhibits a very different appear- 
