43G 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 (irfcrinsus, a muscular cylinder, which, running hori- 

 zontally forward, passes out through the pericardium, and is 

 continued as the less muscular aorta (.4) underneath the 

 branchial arches along the median line ; the aorta gives off 

 branches on both sides, one to each arch to supply the bran- 

 chiae ; the vessels after ramifying are gathered together, to 

 again form a single trunk, which passes backward immedi- 



,B 



Fig. 396 Anatomy of the Gunner, male. L, lateral line ; fft, heart ; G, pharynx ; P, 

 peeuaobranchia ; .Sp, spleen ; S, air-bladder ; 1, Ki', kidneys ; bl, bladder ; T, tes- 

 tis ; A, aorta ; B, brain ; In, intestine ; Z,i, liver ; G, gills. Drawn by C. S. Minot. 



ately underneath the spinal column ; it is called the descend- 

 ing aorta. 



The body and pericardia! 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 tlvj 



