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 

 arise 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 mediums 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 Divisions. The mammals are divided into 

 three sub-classes : i. 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 



