328 COMPARATIVE ZOOLOGY. 
ries a nail, claw, or hoof; the teeth (always present, save 
in certain low tribes) are planted in sockets; the mouth 
Fie. 319.—Longitudinal Section 
of Human Body (theoretical) : 
a, cerebro-spinal nervous sys- 
tem; b, cavity of nose; c, cay- 
ity of mouth; d, alimentary 
canal; ¢, chain of sympathet- 
ic ganglia; f, heart; g, dia- 
phragm. 
is closed by flexible lips; an 
external ear is rarely absent ;'” 
the eyes are always present, 
though rudimentary in some bur- 
rowing animals; they are vivipa- 
rous; and, finally, and perhaps 
above all, while in all other an- 
imals 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- 
ereted by the mammary glands. 
From the first, therefore, till it 
can care for itself, the young 
Mammal is in vital connection 
with the parent.’ 
Fig. 320.—Transverse Section of Human Body 
(theoretical): a, cerebro-spinal nervous axis 
contained in neural tube; e, chain of sympa- 
thetic ganglia; d, alimentary canal; f, heart; 
h, heemal tube. 
1. Monotremes.—This order is created to include two 
singular forms, the Duck-mole (Ornithorhynchus) and 
Spiny Ant-eater (Zchidna), both confined to the Australian 
continent. The former has a covering of fur, a bill like 
