BLOOD-VASCULAR SYSTEM, ETC., OF AMIURUS CATUS. 421} 
wards over the atrium and sinus venosus. These vessels are confined 
to the connective and muscular tissue of the walls, the main stems 
lying wholly in the connective tissue layer. 
THE BRANCHIAL SYSTEM, 
The branchial arteries to the third and fourth arches arise from 
the truncus arteriosus by a single stem which runs backwards and 
upwards to the anterior end of the median ventral ridge of the 
triangular cartilage uniting the fourth and fifth arches. Over this 
it divides into two stems, which immediately divide again, the 
anterior divisions curving forwards and outwards to the third arch, 
and the posterior pair backwards and outwards to the fourth arch. 
This arrangement is not uncommon among Teleosts according to 
Stannius.' The truncus passing forward gives off the arteries to the 
second and first arches, not in pairs, but alternately from the dorsal 
surface, first to the right and then to the left, ending in the left 
stem of the first arch. 
The general features of the branchial arches have already been 
described by Prof. McMurrich in his paper on the osteology of 
Amiurus.” I shall therefore content myself with following the 
course of the blood through them, without attempting a description 
of their histological structure, which has been exhaustively done for 
other Teleosts by Riess,’ Hyrtl, Dréscher,* ec. 
The art. branchiales enter the gills upon the posterior side of the 
arch, nearly 10 mm. from the termination of the filaments which are 
continued forwards upon the membrane, in posterior arches beyond 
the attachment of the adjacent arch. To supply these filaments with 
blood the artery sends back a branch after entering the canal. In 
the canal the branchial artery is placed farthest from the bottom 
of the groove, beneath the rudimentary diaphragm, and gives off a 
branch to each filament of the double row. The artery passes out- 
wards upon the inner side of the filament, while the vein, which 
gathers the blood from the capillaries, returns upon the outer side 
and passes around the branchial artery to enter the branchial vein 
which lies along the bottom of the groove. The branchial nerve lies 
directly between the artery and vein. 
1 Loe. cit., p. 249. 2 Vide, p. 292. 
8 Arch. fiir Nat., 1881, Jahrg. 47, p. 582. 
4 Arch. fiir Nat., 1882, Jharg. 48, Heft. I. & II. 
