286 ELEMENTARY ZOOLOGY. 



materially in vertical steering. The various small fins 

 are continually in small motions. This .probably has to 

 do with the effort to keep the fish upright in the water. 

 Probably the air-bladder, located just under the back- 

 bone has a value in this regard, in those fishes that have 

 it. Its presence tends to lighten the dorsal part of the 

 animal. 



FIG. in. Diagram of the principal vessels in the circulation of the fish, lateral 

 view. Lettering as in preceding figure. Adapted from Parker and Haswell. 



Questions on Figures no and in. Compare the figures and 

 identify every structure common to both. Draw arrows showing 

 the course of the circulation, after studying reference texts. Deter- 

 mine the meaning of each term in the description of these organs 

 and the function of each structure. What is the "hepatic portal" 

 circulation? The "renal portal"? Where is the purest blood in 

 the body? What do you mean by pure? What are the chief 

 impurities removed from the blood in its course? How many 

 regions of capillaries are indicated? What takes place in each? 

 What kind of blood does the heart handle? 



296. Circulation and Respiration. Reference to Figs, 

 no and in will give the student a general idea of the 

 circulation in the fish. The oxygen is received by the 

 blood as it passes through the four or more pairs of gills 

 which lie in the slits that open from the pharynx outward. 

 In most fishes these gills are covered by a shield of bone 



