656 MAMMALIA [CH. 



the orbit. The ducts of all four open into the buccal cavity. Glands 

 in similar positions are found in some Birds, but those of Mammalia 

 secrete in addition to mucus a ferment, called ptyalin, which turns 

 starch into sugar, so that the secretion which is called saliva is 

 a true digestive juice. The large development of the large intestine 

 causes most of the water to be absorbed from the undigested residue 

 of the food, thus reducing it to a semi-solid mass or faeces, which 

 is also a characteristic of the Mammalian alimentary canal. 



Mammals are divided into three great primary divisions or 



sub-classes according to the structure of the ovary and oviduct 



and to the stage of development attained by the 



Classification. . . . . ; , . . , 



young at birth. The lowest forms have comparatively 

 large eggs like those of birds in which abundance of yolk is de- 

 veloped : in the higher forms the egg is at first small but has the 

 power of absorbing nourishment from the wall of the oviduct, which 

 is here enlarged to form a womb or uterus. In the highest 

 division a special organ for the nourishment of the embryo, the 

 placenta, is developed, as an enlargement of the embryonic 

 bladder. 



The sub-classes are called : 



I. PROTOTHERIA, or primitive Mammals. 

 II. METATHERIA, or modified Mammals. 

 III. EUTHERIA, or perfect Mammals. 



Sub-class I. PROTOTHERIA 



The Prototheria include two extraordinary animals, the Or- 

 nithorhynchus (Platypus), or Duck-billed Mole, and the Echidna, or 

 Spiny Ant-eater, which are found only in Australia, New Guinea 

 and Tasmania. In these animals large eggs with a firm shell are 

 laid in a nest and incubated by the mother, and in harmony with 

 this arrangement the two oviducts are large throughout the whole 

 of their length, and do not join each other at any point but open 

 along with the intestine into a common vent or cloaca, as is the 

 case with Birds and Reptiles. The ureters do not open into the 

 bladder as they do in all other Mammalia, but they and the bladder 

 open separately into the cloaca. In the male a copulatory organ or 

 penis is present lying beneath the ventral wall of the cloaca and 

 opening into it in front and protruded from the cloaca behind. 

 After they are hatched the young receive milk from the mother. 



