ZOOLOGY. 



contained structures; the eyes: position, color, lids (is 

 there a nictitating membrane?) ; ears. To what extent is 

 the external ear developed? 



435. The Mammalia embrace, on the whole, the most highly 

 developed vertebrates. To this group man belongs. The birds 

 are more highly specialized in some respects, but the mammals 

 surpass the birds in the size and convolutions of the brain, and 

 in the closer relations between the mother and offspring both 

 before and after birth. The form of parental care seen in the 

 Mammalia is an adaptation resulting in great advantage to the 

 young, and has also produced a great improvement in the 

 mental qualities of the parents. The class contains forms of 

 very varying appearance and perfection of development, and 

 suited to almost every mode of life. Many are aquatic, in- 

 cluding the largest living animals, the whales ; some burrow in 

 the soil, as the mole and many rodents; some live largely in 

 trees, as the monkeys, squirrels, sloths, etc. ; a very few, as 

 the bats, have acquired the power of flight; others the vast 

 majority live on the dry land. 



436. General Characteristics of Mammals. 



1. Mammalia are air-breathing vertebrates in which the 

 covering developed by the epidermis is hair. 



2. In the female, mammary glands occur in the skin, by the 

 secretions of which the young are nourished. 



3. The diaphragm, a muscular partition, completely sepa- 

 rates the body cavity into two, an anterior or thoracic and an 

 abdominal. 



4. With a few exceptions the Mammalia are quadrupeds. 



5. Heart is four-chambered; the temperature of the blood 

 not determined by that of the surrounding medium ; red blood 

 corpuscles not nucleated; one (the left) aortic arch persists. 



6. Two occipital condyles. 



7. Chiefly viviparous (Monotremes are oviparous) ; foetus 

 nourished during early development in the uterus of the 

 mother, often being closely connected therewith by a complex 

 structure known as the placenta. 



