MAMMALIA. 297 
Nervous System.—The brain of mammals is larger than 
that of any of the preceding or lower forms, and extending 
from it is the long, protected cerebro-spinal cord with its 
innumerable nerve-branches. All the impulses of animals 
atise in the brain, that seems to send messages along the 
nerves to the limbs and various organs, and in this way 
action is produced. ‘That the nerves are the médiums of 
communication can be shown by severing them, the part 
so disconnected becoming powerless. 
Organs of Sight, Hearing, etc—The mammals all pos- 
sess eyes, though in the mole they are almost useless. 
With the exception of some seals, the whales, and a few 
others, they have external ears. 
Development.— Except the Monotremes, all mammals 
are viviparous, and differ from all preceding forms in nour- 
ishing their young with the secretion called milk. In some 
Carnivora the young are at first blind and helpless; in 
others, as the herbivorous animals, the young immediately 
follow the parent. The young of marsupials are extremely 
minute and helpless when born. 
General Dtvisions.—The mammals are divided into 
three sub-classes: 1. Ornithodelphia, represented by the 
Monotremes; 2. Didelphia, or the pouched animals; 3. 
Monodelphia, or the placental mammals. 
Sub-Class I.—OrNITHODELPHIA. 
Order I. The Monotremes (Monotremata). General © 
Characteristics —Egg-laying mammals. Ornithorhynchus 
of the Australian region, and Echidna of Australia and 
New Guinea, with flattened or narrow, horny, bird-like 
bills. The eggs are laid at an age equal to a thirty-hour- 
old chick, and are inclosed in a strong, flexible, white shell. 
They measure three fourths of an inch in the long axis, 
and half an inch in the short. One species of Ornitho- 
rhynchus is known, while two distinct forms of the ant- 
eater (Echidna and Acanthoglossus) have been discovered. 
