22 Cincinnati Society of Natural History. 
The efferent gill passage is given off from about the center of 
the convex ouwfer wall. (Fig. X.) The line through these 
openings I shall call the axis of the gill. The internal ana- 
tomy is complex. The mucous membrane of the inner 
walls is folded to form a number of plates which are parallel 
to the axis of the gill and extended radially toward the center. 
(Fig. IX.) There are about twenty large plates, and a large 
number of smaller ones. ‘The latter extend only a short dis- 
tance in from the wall, and fill in the spaces between the 
bases of the other gill plates. Each plate is thrown into 
folds, only moderately near the base, but to an extraordinary 
degree of complexity toward the center. In this way a large 
amount of respiratory surface is developed. ‘The free central 
margin, like the attached base of the plate, is strong and only 
slightly branched. These parts seem to serve merely as 
supports for the extremely thin respiratory leaflets. The 
inner surface of the gill wall, and the thicker supporting parts 
of the gill plates are covered with a stratified epithelium. 
The delicate respiratory leaflets are covered with a thin pave- 
ment epithelium. Next to the epithelial membrane occurs 
a small amount of connective tissue surrounding the blood 
vessels. Externally, the wall of the gill pouch is composed 
of a double layer of striated muscle — the external layer of 
circular fibers, the internal of /ongztudinal fibers (parallel to 
the axis). Surrounding the muscular layer isa thin serous 
membrane, which lines the lymphatic peri-branchial spaces. 
The general distribution of the blood vessels in the gills is 
as follows: The afferent branchial artery of each gill passes 
under the lower margin of each gill pouch, then upward over 
the convex outer gill wall, which it enters just below the 
external gill passage. Some small twigs are given off which 
supply the gill passage and the muscles of the gill wall. The 
afferent artery then divides into two branches which sur- 
round the opening of the gill-passage, forming an irregular 
“ring vessel.”” From this “ring vessel’ several radial vessels, 
often dilated into sinuses, are given off which pass within 
the gill wall toward the peripheral margin. Branches are 
given off which extend along the attached margin of each gill 
plate, beneath the muscular layers of the gill wall. From 
these branches numerous smaller vessels pass directly into 
ice) 
