No.) QU0GUL OF THE TERRESTRIAL ISOPODS. 85 
It will be seen later that no true cell boundaries exist in the 
“midgut” epithelium of adult forms; indications, however, of 
the extent of cytoplasm which comes under the influence of 
each nucleus, if I may use such an expression, are to be found, 
and to each of these “spheres of influence’’ I shall apply the 
term cell. 
The “midgut” in the terrestrial Isopods pursues a practi- 
cally straight course from the stomach to the rectum, and may, 
for convenience, be divided into the four regions which are rec- 
ognized by Ide (92). The most anterior of these regions is 
characterized by possessing along the dorsal mid-line a j-shaped 
ridge which projects into the lumen of the intestine, and may 
be termed the dorsal ridge. The second portion differs from 
the first only in lacking this ridge, while the third portion is 
characterized by being provided with strong circular muscles, 
which act as a sphincter. The fourth portion, which may be 
termed the transitional region, is short, and communicates 
posteriorly with the rectum. 
Throughout the entire length of the gut the general structure 
is the same. Its interior is lined throughout by a moderately 
thick layer of chitin (Pl. IV, Fig. 5, cz), below which is a single 
layer of large cells which produce the chitin and have been 
spoken of as the hypodermis. Beneath this layer is a basement 
membrane (Fig. 5, 4) which separates the hypodermis from 
the mesodermal tissues, muscles (wz), and coelomic cells, which 
form the external layer of the gut. 
In examining a surface preparation of the second portion of 
the gut (Figs. 1, 3), one sees the large nuclei of its epithelium 
arranged in very definite longitudinal and transverse rows, the 
regularity of their arrangement in the longitudinal direction 
being especially striking. The longitudinal rows are continued 
without interruption into the ventral surface and sides of the 
first portion of the gut, but upon the dorsal surface a number 
of the rows on each side of the mid-line converge towards the 
posterior extremity of the dorsal ridge, as is well shown in 
the figure given by Ide ('92, Pl. II, Fig. 19) of a portion of the 
“midgut” of Oxzscus. This figure and the sections which 
the same author represents in Pl. III of his paper show with 
