CHORDATA. 34! 



These changes accompany and are partly caused by the change 

 from gills to lungs. See also the accompanying table. Locate 

 the vessels and trace the changes indicated by means of larger 

 texts. 



351. In fishes the blood passes through the heart only once 

 in making the circuit of the body. In all the air-breathing 

 forms at least a part of it returns twice, passing from the 

 heart to the lung, then back to the heart, and thence to the 

 system and back to the heart again. In amphibia and reptiles 

 the blood from the lung and from the system mix somewhat 

 in the heart, because the partition between the right and left 

 sides is not complete, but in birds and mammals the two sides 

 of the heart are completely separated and the pure and impure 

 blood are not allowed to mix (see Figs. 167, 169). 



352. Excretion. We have seen that carbon dioxid, one 

 of the waste products of the protoplasmic activity, is eliminated 

 through the lungs and skin. Water is similarly excreted. The 

 most important remaining waste (e. g., urea) contains nitro- 

 gen. This is taken from the blood by means of the kidneys, 

 a pair of organs very complicated both as to structure and 

 development. They lie near the middle line of the body at 

 the back of the body cavity. Each gland represents a large 

 number of nephridia or tubules similar in some respects to the 

 segmental organs of worms (Fig. 33), but much more com- 

 plicated. The kidneys are always well supplied with blood. 

 In fishes, amphibians, reptiles, and birds both arteries and veins 

 carry blood to the kidney; in mammals, only arteries. The 

 excretion, more or less in solution in water, is poured by the 

 tubules into a duct the ureter which may be the final outlet ; 

 or the ureters may empty first into a urinary bladder, which 

 has its own outlet (the urethra}. 



353. Reproduction. With a very few exceptions among 

 the fishes the sexes are separate in all the vertebrates. The 

 sexual elements are derived from modified portions of the 

 lining of the body-cavity (germinatwe epithelium'). This 



