CHAPTER XIX 



Mammalia 



Perhaps our chief interest in the Mammals is derived from the 

 fact that we ourselves belong to this great class of vertebrates. In 

 the early days man found his most formidable enemies among the 

 mammals, and even now he suffers from the activities of the rodents. 

 On the other hand, mammals have furnished milk, meat, and furs, 

 and certain forms have been used for centuries as servants and 

 beasts of burden. Like the Birds, they are a tremendous economic 

 factor in our civilization. 



Characteristics 



Mammals have a double occipital condyle, a hairy skin, and well- 

 developed milk glands. They are warm blooded, with the heart 

 divided into four chambers and the single (left) aortic arch curving 

 over the left bronchus and continuing as a dorsal aorta, the visceral 

 arches modified into the earbones, with the cerebral hemispheres 

 usually connected by a heavy commissure called a corpus callosum. 

 The thoracic and abdominal cavities are separated by a muscular 

 diaphragm. 



Order i. Monotremata. — This order includes egg-laying mam- 

 mals with mammary glands devoid of teats. Eggs about half an 

 inch long, rich in yolk with soft, but tough shells. Episternum and 

 coracoid well developed. Oviducts distinct throughout; cloaca in 

 both sexes, into which the ureters and urinary bladder open sepa- 

 rately. In the center of the vertebrae the epiphyses are absent or 

 imperfectly developed. The bones of the skull coalesce early. 

 The corpus callosum is absent. (Figure 210.) 



The spiny ant eater {Echidna aculeata), found in Australia, 

 Tasmania and New Guinea, reaches a length of about 18 inches. 

 As the name would indicate, it is covered with spines and hairs. 

 It curls up like a hedgehog when alarmed. It has no teeth but is 

 provided with a long extensible tongue which it utilizes in securing 

 insects and worms. It places its egg in a primitive mammary pouch 

 consisting of a fold of the abdominal skin. The young when hatched 



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