The Tetrapoda: Mammals 629 



The circulatory system is highly efficient as is necessary in the 

 maintenance of homoiothermy. The heart is four-chambered, and there 

 is complete separation of oxygenated and unoxygenated blood. Only 

 the left fourth aortic arch persists as the systemic arch ; the sixth aortic 

 arches serve as pulmonary arteries. All respiration is through the 

 lungs. The inspiration and expiration of air is accomplished by changes 

 of pressure in the chest cavity mediated by the intercostal muscles and 

 the muscular diaphragm. This diaphragm, separating the thoracic and 

 abdominal cavities, is a unique development with the mammals. 



The nervous system is highly developed with a large complex brain. 

 Characteristic of the mammalian brain is the tremendous enlargement 

 of the cerebrum. The optic lobes are subdivided into the corpora 

 quadrigemina. 



The kidney is of the metanephric type, but has many more tubules 

 than that of the reptiles. Urinary wastes are stored in the urinary 

 bladder until voided. 



Fertilization is internal in all forms, and the male has a copulatory 

 organ, the penis, for transference of the sperm to the female vagina. 

 The eggs are usually minute and with virtually no yolk. Except for 

 the Prototheria, development of the young takes place in the uterus 

 of the female. Among the marsupials, the period of intrauterine de- 

 velopment is very brief, and the "embryos" continue their develop- 

 ment in the marsupium, an external pouch on the belly of the female. 

 Among the Eutheria, the embryonic membranes are variously modified, 

 and the developing fetus is nourished through the placental attach- 

 ment. After birth, the young are nourished by the secretions of the 

 mammary glands. 



The Classification of the Mammals. — All the approximately 

 15,000 species of living mammals are grouped into some eighteen 

 orders. The separations are based on various structural characters such 

 as skull, teeth, limb modifications, and special structures. 



Subclass Prototheria. 



Order 1. Monotremata. Examples: duckbills, Ornithorhynchus; spiny "ant- 

 eaters," Tachyglossus and Zaglossus. 



Subclass Theria. 



Infraclass Mctatheria. 



Order 1. Marsupialia. Examples: Virginia opossum, Didclphis virginiana; 

 numerous marsupials of Australia. 



