VERTEBUATA. 



341 



a few 

 riee a 



aquatic forms ; excepting the Whales, each digit car- 

 nail, claw, or hoof ; the teeth (always present, save 

 in certain low tribes) are planted in 

 sockets ; the mouth is closed by flexi- 

 ble lips ; an external ear is rarely ab- 

 sent ;'" the eyes are always present, 

 though rudimentary in some burrow- 

 ing animals ; they are viviparous ; 

 and, finally, and perhaps above all, 

 while in all other animals the embryo 

 is developed from the nourishment 

 laid up in the egg itself, in Mammals 

 it draws its support, almost from 

 the beginning, directly from the 

 parent, and, after birth, it is sus- 

 tained for a time by the milk se- 

 creted by the mammary glands. 

 From the first, therefore, till it can 

 care for itself, the young Mam- 

 mal is in vital connection with the 

 parent. 



Fia. 329.— Lougitndinal Section 

 of Human Body (tlieoretical) : 

 a, cerebro-epiiial uervons sys- 

 tem ; b, cavity of nose ; c, cav- 

 ity ofjnouth; d, alimeutaiy 

 canal ; e, chain of sympathet- 

 ic ganglia; /, heart; g, dia- 

 phragm. 



Fio. 330. —Transverse Section of Human Body 

 (theoretical) : a, cerebro- spinal nervous axis 

 contained in neural tube ; e, chain of sympa- 

 thetic ganglia; d, alimentary canal; /.heart; 

 h, haemal tube. 



Subclass I. — Ornithodelphia. 

 These Mammals have but one outlet for the intestine, 

 urinary and reproductive organs, as in Birds. They are 

 implacental. There is but one order. 



