206 ECONOMIC ZOOLOGY 



hibernate in a dormant condition in some hidden place and thus pass 

 the cold, foodless season, their warm blood and covering of hair pro- 

 tecting them against the cold. The red blood corpuscles of almost all 

 mammals are circular discs, without nuclei. In all but two or three 

 small groups all mammals have seven cervical vertebrae. 



The number of known species of living mammals is variously stated. 

 Henshaw, in 1912, estimated the number at 7000; other authorities 

 place the number even higher. 



The classification of the Mammalia, unlike that of the birds is 

 pretty well established. By most authors they are divided into two 

 subclasses: (i) Prototheria or egg-laying mammals and (2) Eutheria 

 or viviparous mammals. 



Under the first subclass is included but one order, the Monotremata ; 

 under the second subclass fall all the higher mammals including, of 

 course, man. 



As has been said this is economically the most important of all the 

 groups of animals, though there is very great variation in the relative 

 importance of the orders. These orders will now be taken up in turn 

 but not in order of their economic importance. 



Monotremata. Duckbill and Spiny Anteater. These animals are 

 commonly known as the egg-laying mammals from the fact that they 

 lay eggs like those of birds and reptiles, the young being nourished by 

 the yolk of the egg instead of through the placenta, as in other mam- 

 mals. They are small animals, 18 to 24 inches in length, found only 

 in the region of Australia and New Zealand. Their name is derived 

 from the fact that the urogenital organs and intestine open into one 

 cavity, the cloaca, as in birds and reptiles; they have other reptilian 

 characteristics, also. The presence of hair and rudimentary mammary 

 glands causes them to be classed as mammals. The two best known 

 species of this small order are the duckbill or platypus, Ornithorhynchus , 

 and the spiny ant-eater, Echidna. These forms, though most in- 

 teresting zoologically, have but little economic importance. The 

 former doubtless is of value as an insect eater, the latter is used for 

 food to some extent. 



Marsupialia. Kangaroos and Opossums. The chief character- 

 istic of this order and the one from which the name is derived is the 

 possession of a marsupium or pouch of skin on the ventral abdominal 

 wall; in this pouch the young, which are born in a very immature 



