CHORDATA 



consist of a pair of kidneys lying in the abdominal region, wh< 



ducts, the ureters, open into the cloaca. I'lu-re is no urinary 

 bladder, and the secretion from the kidneys is not a fluid, bul 

 whitish, semifluid substance. 



The reproductive organs in the male arc a pair of testes lying 

 within the body-cavity, from which numerous fine tubes pass off 

 into one duct for each testis, — the vas deferens, and the two vasa 

 defireniia open into the cloaca. There is no copulatory organ, 

 as a rule, although a rudimentary one is found in some birds, 

 and in the ostriches there is a single penis developed on the wall 

 of the cloaca very much like that in the Chelonia and Crocodilia. 

 In the female the right ovary and oviduct are very rudimentary 

 or absent ; the left is alone functional and the oviduct consists 

 of several regions. The eggs are fertilized in the anterior por- 

 tion of the tube, and then as they pass toward the cloaca they 

 are surrounded by the albumen, the shell membranes, and finally 

 by the calcareous shell. 



All the birds are oviparous. The number of eggs varies 

 from one to many in different species. Heat is necessary for 

 incubation, and this is generally secured by the bird's sitting 

 on the eggs. In some cases it is the female alone that sits. 

 in some cases the male and female alternate with each other 

 at pretty definite intervals. Most birds are monogamous, and 

 most build nests ; usually it is the female only that builds 

 the nest; in some cases the male assists in bringing the mate- 

 rial for it, in others he takes no part whatever in the work. 

 The nests are often very elaborate structures, but some are a 

 loosely arranged mass of sticks, a hole in the ground, or a shallow 

 excavation in the sand, as in the case of the ostriches. The 

 habits of the European cuckoo and the American cowbird are 

 well known ; they build no nest and do not sit upon their eggs, 

 hut deposit them in the nests of other birds, which incubate 

 them and feed the young. The period of incubation varies 

 greatly, from eleven or twelve days in the case of the humming 

 birds to seven or eight weeks in the case of the ostriches. In 

 their development an amnion and allantois are formed as in the 

 reptiles. In different species the young are born in very differ- 

 ent stages of development. In some, as in the domestic fowl, 

 the young when hatched are covered with down, have their eyes 



