CHORDATA — VERTEBRATA 335 



tance respectively. The eyeball is kept distended by the large 

 mass of vitreous humor and by the smaller aqueous humor. 



In vertebrates below the mammals, and even in the Mono- 

 tremata, the sclerotic coat is more or less cartilaginous, and 

 within it many species have developed a ring of delicate, bony 

 plates near the junction with the cornea ; these sclerotic 

 plates surrounding the orbit of the eye are especially con- 

 spicuous in the extinct Stegocephalia and Ichthyosauria, in 

 lizards, chelonians, and modern birds (Fig. 159, scL). 



The organ of hearing is similarly a complicated apparatus. 

 As in the invertebrates it acts Ukewise in the maintenance of 

 equilibrium, though in the more highly organized vertebrate 

 only a portion of the hearing mechanism, the semi-circular 

 canals, performs this function. The sense of smell is lodged 

 in the nasal cavity, that of taste chiefly in the tongue, while the 

 surface of the entire body is an organ of touch. 



Cats are of two sexes. The ova are fertilized within the 

 female by the spermatozoa introduced by the male. Soon 

 each developing embryo, the foetus, becomes attached to the 

 walls of the uterus by a special structure, the placenta. Through 

 this it is nourished for fifty-five or fifty-six days. The young 

 are then produced, usually five or six at a birth, fully formed 

 except that the eyelids are still closed and the hairy covering 

 is incomplete. The young before birth are enveloped in several 

 foetal membranes. The inner one, called the amnion, is similar 

 in the reptiles and birds. Because of its presence in the three 

 groups, the reptiles, birds, and mammals are sometimes classed 

 together as the Amniota. After birth the young of the cat 

 are nourished by milk, which is secreted by skin glands known as 

 breasts or mammae. Because of the possession of these glands the 

 group of animals to \yhich the cat belongs is called Mammalia. 



1. Examine the mounted skeleton, identifying the various 

 bones by means of the illustration and description. 



2. How is the cat protected against changes in cUmate? 

 from enemies ? 



