MAMMALIA 439 



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

 the external ear developed ? 

 III. Internal Structures. 



(The rat, rabbit, or cat will serve if it is desired to dissect a 

 mammal) . The student should map out his own laboratory 

 outline by reference to descriptive texts. Two fundamental 

 questions should be kept in mind in all such work : How do 

 the discovered structures compare with those studied in 

 other animals, particularly the vertebrates ? What service 

 do they render the animal and how well are they adapted to 

 do the work put upon them? Make a record both by 

 accurate description and by drawings. 



446. 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. 



447. General Characteristics of Mammals. 



1 . Air-breathing vertebrates in which the covering developed 

 by the epidermis is hair. 



2. Mammary glands in the skin of the female, by the secre- 

 tions of which the young are nourished. 



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

 rating the body cavity into two, a thoracic and an abdominal. 



4. Quadrupeds, with a few exceptions. 



5. Heart four-chambered; temperature of the blood not 



