244 



BIOLOGY FOR BEGINNERS 



making only two loops, and opening into it is the duct from a well- 

 developed liver between whose lobes the gall sac can be found. 



Circulation. The fish has a heart consisting of two chambers, 

 an auricle and a ventricle, located just posterior to the isthmus. 

 So it is literally true that its " heart is in its throat." The blood 

 leaves the heart by a large artery that branches to each of the 

 gills, in whose filaments it is relieved of its carbon dioxide. Then 

 laden with oxygen it flows into a dorsal artery with branches to 



all the muscles and in- 



'"""""" *"-" ternal organs where it 



exchanges this oxygen 

 for carbon dioxide. The 

 blood which flows to the 

 digestive organs receives 

 the digested food-stuffs 

 which they have pre- 

 pared, and passes 

 through the liver and 

 so back to the auricle of 

 the heart. Thus it hap- 

 pens that the heart is always pumping blood that is rich in nutrients 

 and carbon dioxide but poor in oxygen. The course of the blood 

 stream is from the ventricle of the heart, to gills, to general circu- 

 lation and digestive organs, to liver, and back to auricle of the 

 heart again, though a part passes through the kidneys each time, 

 where urea and other wastes are removed. 



Nervous System. The central nervous system in all vertebrates 

 is located in the dorsal body cavity, protected by outgrowths from 

 the spinal column. This arrangement is entirely different from 

 that found in the invertebrates, where the nervous system lies 

 along the ventral side and is not separated from the other internal 

 organs. 



In the case of most fishes the nervous system consists of the 

 spinal cord, extending the whole length of the body, protected by 

 arches of bone attached to each vertebra. From it many nerves 

 extend to the muscles and internal organs. At the anterior, the 



FIG. 87. Diagram of circulation in fish. 



