344 ZOOLOGY 



of the heart are completely separated and the pure and impure 

 blood are not allowed to mix (see Figs. 169, 170). 



360. 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 nitrogen. 

 This is taken from the blood by means of the kidneys, a pair 

 of organs very complicated both as to structure and develop- 

 ment. 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 seg- 

 mental organs of worms (Fig. 35), but much more compli- 

 cated. 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). 



361. 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 (germinative epithelium). This layer, 

 supported by connective tissue, forms the essential part of 

 the ovaries and testes, of which there is usually a single pair. 

 The eggs vary in size from Jf oo of an inch in mammals to 5 

 inches (ostrich), or more in some extinct birds. The outlets 

 for the ova and spermatozoa (oviducts and vasa dejerentia) are 

 modified portions of the embryonic excretory and kidney ducts. 

 Throughout the group there is a close connection between the 

 excretory and the reproductive organs. The oviducts may have 

 special glands for depositing nutritive or protective material 

 about the egg before or after fertilization (as the albumen and 

 shell in egg of birds). Fertilization is external in most fishes 

 and some amphibia, and internal in the higher groups. Natur- 

 ally it must be internal in all forms in which large masses of food 

 or shells come to surround the ovum proper, since the sperm 



