ITS ANIMAL STUDIES 



than in any of the invertebrates. It also differs in its gen- 

 eral plan from that of most adult vertebrates, owing to the 

 peculiar method of respiration. In almost every case the 

 vessels returning from all parts of the body unite into one 

 vein leading into the heart, which consists of only one 

 auricle and ventricle (Fig. 107). From the heart the blood 



FIG. 107. Dissection of a bony fish, the trout (Salmo). a.bl., air-bladder ; an., anal 

 opening ; au., auricle ; gl.st., gills ; gul., esophagus ; int., intestine ; kd., kidney; 

 lr., liver ; l.ov,, ovary ; opt.L, brain ; py.c., pyloric cceca ; sp.c., spinal cord ; spl., 

 spleen ; st., stomach ; v, ventricle. 



is forced through the gills, with all their delicate filaments, 

 and now, laden with oxygen and nutritious substances, al- 

 ready absorbed from the coats of the digestive tract, it 

 travels on to all parts of the body, continually unloading 

 its cargo in needy districts and waste matters in the kid- 

 neys before returning once more to the heart. 



169. The senses of fishes. The habits of fishes indicate 

 that they know considerable of what is going on in the out- 

 side world, and their well-developed sense-organs show the 

 degree of their sensitiveness. A share of this information 

 comes through the sense of touch, which is distributed all 

 over the surface of the body, chiefly in the more exposed 

 regions sometimes especially provided with fleshy feelers, 

 like those on the chin of the catfish. 



