VERTEBRATA. 
341 
a few aquatic forms; excepting the Whales, each digit car¬ 
ries a 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 ; 174 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. 
a h 
Pig. 329.—Longitudinal Section 
of Human Body (theoretical): 
a, cerebro-spinal nervous sys¬ 
tem ; 6, cavity of nose; e, cav- Fio. 330. —Transverse Section of Human Body 
ity of mouth; d, alimentary (theoretical): a, cerebro-spinal nervous axis 
canal; e, chain of sympathet- contained in neural tube; e, chain of sympa- 
ic ganglia; /, heart; g, dia- thetic ganglia; d, alimentary canal; /, heart; 
phragm. 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. 
