86 MCMURRICH. [VOL. XIV. 
sufficient accuracy the structure of the dorsal ridge. In the 
sphincter region the epithelial cells seem at first sight to differ 
considerably from those of the more anterior regions; they are 
somewhat more columnar, and the entire epithelium is thrown 
into numerous folds. These peculiarities, however, are proba- 
bly to be explained by the presence of the strong sphincter 
muscle, which diminishes materially the lumen of the gut, 
pressing the epithelial cells together and throwing the layer 
into folds. In fact, it seems probable that the actual surface 
of the epithelial layer of a given length of this portion of the 
gut is equal to that of a similar length of the second portion, 
an idea which is borne out by the fact that in very young 
specimens, such as have been but recently set free from the 
brood pouch, no difference can be detected between the posterior 
and anterior portions of the gut, the cell rows being continuous 
from the rectum forward. 
I have not paid much attention to the transitional region, 
and can only say that its cells seem to resemble closely those 
of the second portion of the gut, and diminish in size posteriorly 
rather abruptly to become continuous with the small-celled 
epithelium of the rectum. 
From this brief sketch of the general structure of the ‘“ mid- 
gut,” we may now pass to a detailed consideration of the 
structure of its epithelial cells. 
The layer of chitin, which covers the inner surface of the 
epithelium, consists of two layers (Fig. 5), an external denser 
and more refractive layer (ch'), which stains deeply with Hai- 
denhain’s haematoxylin, and a less dense inner layer (cz), which 
rests on the surface of the epithelial cells and remains unstained 
in preparations treated with the iron-lack hematoxylin. oth 
these layers are perfectly homogeneous, showing no trace of pores, 
or even of a vadial structure. 
The basement membrane (6zz), on which the epithelium rests, 
is very thin, and stains with the iron-lack hematoxylin. For 
the most part it is of very even thickness, but opposite the 
lines of junction of adjoining cells it is very frequently thick- 
ened somewhat, and below the elongated cells which are found 
on either side of the dorsal ridge, small thickenings are to be 
