32 ZOOLOGY. 



and porcupine the hairs take the form of spines. The 

 armadilloes are partly covered with bony plates. 



32. The members of this group are usually arranged in 

 two* great divisions or sub-classes : 



1. Placental mammals. 



2. Non-placental mammals. 



The placenta (Lat. placenta, a cake) is a structure de- 

 veloped by the allantois, and composed of vascular tissue. 

 The young of mammals, possessing this placenta, remain 

 for a considerable period within the body of the mother, 

 where they are nourished by arterial blood supplied 

 through the minute vessels of this organ. In the iion- 

 placental mammals the young are brought forth much 

 earlier ; and, not having been nourished by a placenta, 

 are born in a very imperfect condition. They are then 

 placed by the mother within a "pouch" beneath the 

 abdomen, where they become attached to the nipple; and, 

 being unable to obtain the milk by suction, it is forced 

 into their mouths by a special muscle connected with the 

 mammary gland. 



CLASS II. Aves. 



33. Aves (Lat. avis, a bird) or birds form the second 

 class of vertebrate animals. 



As already stated, birds have been placed along with 

 reptiles in the section of the Vertebrata which has been 

 termed Sauropsida. The points of agreement are : At 

 no period of life are gills present ; the skull is joined to 

 the vertebral column by a single condyle on the occipital 

 bone ; the lower jaw is composed of several pieces, and is 

 united to the skull by a quadrate bone ; the red blood- 



* Professor Huxley, following De Blainville, divides mammals 

 into three sub-classes : 



1. Ornithodelphia, which embraces the order Monotremata. 



2. Didelphia, which corresponds to the Marsupialia. 



3. Monodelphia, which includes all other mammals. (See Hux- 

 ley's Classification of Animals. ) 



