AVES 271 



Excretory System. — Paired tri-lobed kidneys empty by means 

 of ureters directly into the cloaca. Fseces and urine are given off 

 mixed. The bird kidney is a metanephros. 



Reproductive System. — Testes are oval and are situated dorsally 

 along the back. Each testis has a vas deferens leading to a seminal 

 vesicle, where sperm is stored. In copulation sperm is simply trans- 

 ferred from the cloaca of the male to that of the female; for there is 

 no penis in most birds. The left ovary only is functional, the right be- 

 coming vestigial early in development. The single oviduct is large and 

 complex, and is provided with albuminous and shell glands. The egg, 

 which is what is popularly called the yolk, is fertilized before it de- 

 scends very far into the oviduct, soon becomes wrapped round with 

 layers of albumen and, before it is laid, is covered by a shell which is 

 secreted by glands in the lower part of the oviduct. Eggs are incu- 

 bated outside of the body usually by means of the body heat of one 

 or both parents. 



Nervous System (Fig. 146). — The brain is very short and broad. 

 The cerebrum is large but not convoluted; the cerebellum is very large 

 and complex; optic lobes are well developed; olfactory lobes, rudi- 

 mentary, indicating poor sense of smell. 



Sense Organs. — The olfactory epithelium is poorly developed; 

 sense of taste is almost as poorly developed as the olfactory. The in- 

 ner ear, especially the cochlea, is more complex than in reptiles. The 

 eye is the bird's main dependence; it is large and highly organized, 

 probably keener than that of any other animal. Sclerotic plates cover 

 the eye-ball. A fan-shaped pecten (absent in Apteryges) of unknown 

 function is^ suspended in the vitreous humor. 



The Origin of Birds 



About the origin of birds palaeontology says but little. Only one 

 link definitely connecting the true birds with their reptilian ancestry 

 has been discovered. This one link is the bird-reptile Archosopteryx, 

 a form distinctly intermediate between the bird and the reptile, about 

 which we shall have more to say in other connections. The evolu- 

 tion of modern avian characters from those seen in Archceopteryx is 

 easy to imagine, for all of the avian characters are foreshadowed in 

 this creature, which though more reptile-like than any other bird, is 

 really not a reptile but a bird. What we need to find is a pro-avian 

 ancestor of the birds, some true reptile that exhibits unquestioned 



