26G 



ANATOMY OF VERTEBRATES. 



CHAPTER XXV. 



CHARACTERS AND TRIMARY GROUPS OF THE CLASS 

 MAMMALIA. 



§ 172 

 guished 



138 



. Class Characters. — Mammals are outwardly distin- 

 by a covering of hair, entire or partial, and (with two ex- 

 ceptions) by teats, fig. 138, «, h, whence the 

 name of the class. ^ All possess mammary 

 glands and suckle the young : the embryo or 

 foetus is developed in the womb. The male 

 has a penis, and impregnation is preceded 

 by intromission. The lungs, fig. 139, Ig, 

 minutely cellular throughout, are sus- 

 pended freely in a thoracic cavity separated 

 by a musculo-tendinous partition or ' dia- 

 phragm,' ib. d, from the abdomen. 



Mammals, like Birds, have a heart, ib. 

 h, composed of two ventricles and two auricles, and have warm 

 blood : they breathe quickly ; but inspiration is performed chiefly 



New-born foetus and teats, Kan- 

 garoo {Macropus major). 



139 



ZV.^ 



Ideal section of a Mammalian animal. 



by the agency of the diaphragm ; and the inspired air acts only 

 on the capillaries of the pulmonary circulation. 



' Mamma, a teat. The Monotrcmes have mammary glands without teats. The 

 fcctal Cctacea show tufts of hair on the muzzle. 



