158 



Descriptive Zoology. 



the sinus the blood enters the auricle, which also is thin- 

 walled ; from the auricle it passes into the ventricle, whose 

 walls are thick and muscular, and by whose contraction the 

 blood is pumped clear around the whole circuit to the heart 

 again. From the anterior end of the ventricle runs for- 

 ward the artery leading to the gills. The first part of the 

 artery is often enlarged and is sometimes called the arterial 

 cone or arterial bulb. This artery divides into four branches 

 on each side, one to each gill. After traversing the gills, 

 the blood-tubes (still called arteries) unite on each side, and 

 later the two arteries thus formed unite to form one dorsal 

 artery which- supplies all parts of the body. The small 

 arteries subdivide and form capillaries, which pervade all 

 the tissues. The capillaries unite to form the veins, which 

 again bring blood to the heart. 



How the Perch Breathes. When watching a live fish 

 one sees that the mouth and gill openings open and close 

 alternately. It can easily be proved that water enters the 

 mouth and passes out through the gill openings ; thus a 

 pretty constant current of water flows over the gills. Each 

 gill consists of a bony arch, hinged at the upper and lower 

 ends and jointed in the middle. Along the posterior border 

 of each gill is a red fringe, the red color being due to the 

 red blood within, which shows through the thin, delicate 

 coverings of the gill filaments, as the individual parts of 

 the fringe are called. As the blood comes up into the 

 gill from the artery below, it goes off into small side 

 branches running out into the filaments ; when it returns 

 along the other margin of the gill filament, it enters 

 another artery to pass out at the upper end of the gill. 

 Thus it is clear that there is a constant flow of blood in 

 very narrow, thin-walled tubes in the thin-covered gill 

 filaments; there is also a stream of fresh water flowing 



