THE GRAY SQUIRREL 409 



lungs are larger and more extensible than in the birds and 

 hang free in the thoracic cavity. Respiration is effected 

 mainly by movements of the diaphragm and the ribs, thus 

 altering the size of the thoracic cavity and causing air to 

 enter and to leave the lungs. 



The Excretory System. The kidneys (Fig. 215) are bean- 

 shaped bodies in the dorsal part of the abdominal cavity. 

 The ureters lead from them to the urinary bladder, whence 

 the waste products are carried to the surface by the urethra. 



The Skeletal System. The skeleton is, in general, built 

 upon the plan with which we have become familiar in the 

 study of the frog, the lizard, and the pigeon. The cranium 

 (Fig. 215) is articulated to the vertebrae by two condyles, as 

 in the amphibians. All the vertebrse, except those in the 

 pelvic region, are free. 



The Nervous and Muscular Systems. The nervous system 

 is similar to that of the bird, but the cerebrum (Fig. 215) is 

 considerably more differentiated. The muscles, especially 

 those in the hind legs, form a complex system adapted to 

 strong and rapid movement. 



The Reproductive System. The organs of reproduction in 

 the male consist of oval spermaries (Fig. 215) and a penis ; 

 in the female, of ovaries with their oviducts. 



Development. The squirrel is viviparous. The ovum pro- 

 duced in the ovary passes into the oviduct, where it becomes 

 fertilized. In that portion of the duct called the uterus, it 

 develops into the young squirrel. The young is born in 

 a condition resembling the adult, though the hair is not com- 

 pletely formed and the eyes are closed. In the Southern 

 states the first of two or three litters appears early in March, 

 and four young are usually produced at a birth. The nest 

 is usually in a hollow tree in the colder portions of the squir- 

 rel's range, and exposed on the branches in the warmer 

 regions. The female keeps the male away from the young 



