THE ANIMAL KINGDOM 



19 



Sub-Class Marsi'itals 



The marsupials or didelphiansgive birth to their young in a most imma- 

 ture state and nourish them for some time in an external marsupial pouch 

 situated on the ventral side of the body of the female. The brain has no 

 corpus callosum. A loose allantoic placenta occurs in some. Dasyurus 

 has a yolk-sac placenta. 



Opossum and kangaroo are well-known examples. All the indigenous 

 mammals of Australia are non-placental. 



Sub-Class Placentals 



The placentals or monodelphians have a placenta, a corpus callosum in 

 the brain, and no marsupial bones. Urogenital and digestive outlets are 

 separated. 



Placentals are subdivided into at least ten 

 living orders. 



Order I. Insectivora. The insectivores 

 include shrews, moles, and hedgehogs. They 

 are flat-footed and five-toed, and their denti- 

 tion is unspecialized, so that they are appar- 

 ently nearest of surviving forms to the 

 original placental. 



Order 2. Xenarthra. The Xenarthra 

 include part of the group formerly included 

 in the edentates such as the armadillos, sloths 

 and anteaters. The teeth of adults are either 

 absent or lack enamel and roots. Dentition 

 is limited to a single set. 



Order 3. Rodentia. The rodents are 

 gnawing animals, such as rats, rabbits, 

 squirrels, guinea pigs, beavers, porcupines, 

 gophers. Canine teeth are absent, and the 

 incisor teeth in both jaws grow continuously Fig. 

 throughout life. The cecum is very large. 



Order 4. Carnivora. The carnivora include the fossil creodonts, 

 the cats, dogs, weasels, bears, raccoons, and seals. Each foot has four or 

 five toes. The canine teeth are sharp and elongated. The clavicle is 

 reduced or absent. 



Order 5. Artiodactyla. Artiodactyls are such hoofed forms as 

 cattle, deer, swine, sheep, goats, camels, llamas, hippopotamuses, and 

 giraffes, which usually have an even number of toes on each foot. The 

 third and fourth toes are larger, and the second and fifth reduced or absent. 

 The stornach is complex and the cecum reduced. 



22. — Tupaia, the tree- 

 shrew, an insectivore. 



