54 INVERTEBRATE ZOOLOGY. 



the "milk" is a secretion of perspiratory 

 glands, richly developed in a certain area, 

 the mammary jiocket, and thns not 

 strictly homologous with the milk of 

 other mammals. Alimentary canal, 

 urethra, and reproductive dncts open into 

 a common cloaca. 

 •if Ornithorhynchus, Echidna. 



Sub-class II. Marsupialia Viviparoiis mammals without placenta. 



The young are born in a very immature 

 condition and brooded in an external ab- 

 dominal pouch, the marsupium. Two 

 lateral uteri imperfectly united. Divi- 

 sion between anus and sinus iiro-genitalis 

 internal and indistinct. 



Order 1. Zoophagi Carnivorous marsupials with pointed 



teeth and well developed canines. 

 ■if Didelphys. 



Order 3. Phy tophagi Herbivorous marsupials with flat teeth 



and reduction of canines. -^ Macropufi. 



Sub-class III. Placentalia Young nourished in uterus of mother by 



a capillary mass, the placenta, which ad- 

 heres to the uterine wall and is connected 

 with the embryo by the umbilical cord. 

 Anus separated from the uro-genital 

 sinus by aperinaeinii. 



Order 1. Edentata Teeth either wanting or in condition of 



retrogressive metamorphosis. Incisors 

 and canines generally fail. Large num- 

 ber of sacral vertebrae. -^ Dasypus. 



Order 2. Cetacea Aquatic forms with naked skin, provided 



with hair in embryo. Hind limbs fail 

 externally. Rudiments under skin. 

 Mammary glands in folds on the sides of 

 the vagina. External nares in top of head 

 connecting directly with a tubular pro- 

 longation of the larynx. -^ Balaena. 



