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. 1 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, 3 Hyrtl, Drbscher, 4 &c. 



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 membx^ane, 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. 



i hoc. Kit., p. 240. 2 Vide, p. 292. 



? < Arch, fur Nat., 1881, Jahrg, 47, p. 582. 

 * Arch, fur Nat.. 1882, Jharg. 4S, Heft. I. & II. 



