132 



ANIMAL BIOLOGY 



FIG. 99. Diagram of the circula- 

 tion of a fish. 



The blood passes from the 

 capillaries into the veins, 1 

 which are thinner-walled than 

 the arteries. These veins carry 

 the blood back to the heart. 

 The heart (Fig. 100) consists of 

 two principal parts; a thin- 

 walled auricle which receives 

 blood from the veins, and a 

 thick-walled, muscular portion 

 called the ventricle, which 

 forces the blood out into the 

 arteries. 



100. Adaptations for breath- 

 ing. Laboratory study. 



1. Raise the gill covers of a 



preserved fish and find 

 the gills. Carefully 

 separate the gills with 

 the forceps. How 

 many gills are present 

 on each side? 



2. The openings between the 



gills are called gill 

 clefts. Gently push a 

 thin strip of wood or 

 the forceps through 

 one of the gill clefts 

 as far as you can. 

 a. In what cavity do the 

 forceps or strip of 

 wood appear ? 



1 This is true of all organs of 

 the fish, excepting the gills. See 

 101. 



