436 ZOOLOGY. 
The walls of the auricle are comparatively thin ; the auriculo-. 
ventricular orifice is provided with valves, which prevent 
the blood flowing back into the auricle. The walls of the 
ventricle are thick and very muscular ; from the upper end 
of the ventricle close to the base of the auricle springs the 
bulbus arteriosus, a muscular cylinder, which, running hori- 
zontally forward, passes out through the pericardium, and is 
continued as the less muscular aorta (A) underneath the 
branchial arches along the median line ; the aorta gives off 
branches on both sides, one to each arch to supply the bran- 
chiz ; the vessels after ramifying are gathered together, to 
again form a single trunk, which passes backward immedi- 
Fig, 396 —Anatomy of the Cunner, male. Z, lateral line ; H¢, heart; G, pharynx : Ps, 
pseudobranchia ; Sp, spleen ; S, air-bladder ; Ki, Ki’, kidneys ;'b/, bladder; 7, tes- 
tis ; A, aorta; B, brain ; Zn, intestine ; Zi, liver 3 @, gills—Drawn by C. S. Minot. 
ately underneath the spinal column ; it is called the descend- 
ing aorta. 
The body and pericardial cavities are called serous, because 
their lining membranes are always moist with serum, a watery 
fluid, very much like blood-plasma. The lining of the body- 
cavity is named the peritoneum, and forms a continuous cov- 
ering around the viscera. It is important to observe that the 
various organs simply project into the body-cavity and do 
not lie really inside of it. In fishes we find the disposition of 
the parts to correspond more closely with the fundamental 
type of Vertebrate structure than it does in higher forms, in 
which further modifications have supervened. The pharynx 
still has its distinctive character ; the pericardium lies at the 
