366 



ZOOLOGY 



384. Supplementary Exercise for the Library. — What is the structure and 

 position of the "swim-" or air-bladder in fishes? With what organ is it related? 

 Does it communicate with the outside? Are there any evidences that it is of 

 value as a respiratory organ in any of the fishes? Can you conceive any use it 

 might be in steering, for the purpose of rising or sinking in the water? What 

 would be the effect of compressing the air-bladder at one end more than at the 

 other? 



385. The Circulation. — Little needs be said here in addition 

 to what has been said in the general discussion of the vertebrate 

 circulation (see Figs. 16 6-1 71). The heart is two-chambered. 

 The auricle receives the venous blood from the system; it is 

 passed to the ventricle through a valve which forbids its passage 

 in the reverse direction. From the ventricle the blood passes 

 through a valvular region into the ventral aorta, which carries it, 

 by a series of right and left branches, to the gills. Here aeration 



Fig. 182. 



cu.l. 



Pig. 182. Diagram of the principal vessels in tlie circulation of a Fish, — lateral view. Letterign 

 as in the preceding figure. Adapted from Parker and Haswell. 



Questions on the figure. — Compare the two views (Pigs. i8i and 182) and 

 identify the parts common to both, tracing the course of the circulation in the 

 various vessels. 



takes place, the pure blood being gathered from the gills by a 

 series of efferent branches which combine (except some anterior 

 branches which go to the head) to form a dorsal aorta. The 

 dorsal aorta gives off branches to the body wall, to the paired 

 appendages, to the liver, digestive tract and kidneys, — con- 

 tinuing into the tail where it breaks up in the muscles. 



The impure blood from the capillaries of the tail is brought 

 back to the kidneys by the renal portal vein, where it again 

 passes through capillaries; here the blood is purified of its urea 



