MAMMALIA 



461 



stand apart from the rest. Their young are born in a very 

 immature state and carried by the mother for some time 

 in a pouch under the belly. They have a double vagina, 

 and the anus and urinogenital openings are surrounded 

 by a common sphincter. They are found principally in 

 Australia, where they form almost the whole of the wild 

 mammalian population, including the Kangaroos, Wombats, 



Fig. 336. — Diagrams of the female organs of generation : 



A, of a duckmole (Monotremata) ; B, of a kangaroo (Marsupialia) ; C, of a rabbit 

 (Eutheria). 



cl. t Cloaca ; cl'., vestige of cloaca represented by common sphincter around opening 

 of vestibule and anus; F.t., Fallopian tube (oviduct); <?., internal opening of 

 oviduct ; part., position in which a passage breaks through for the birth of the 

 young; ut., uterus; vag,, vagina; vag.c, caecum of vagina;; vest., vestibule. 



Note; in A no vagina, in B double vagina, in C single vagina. A further 

 development is seen in man (Fig. 355), where there is a single uterus. Note also 

 the progressive disappearance of the cloaca from A to C. 



etc., but the Opossums of America also belong to this 

 group. The remaining mammals, constituting most of the 

 class, are known as Eutheria. 



The most aberrant of the Eutheria are the Cetacea or 

 Whales and Dolphins — purely aquatic creatures 

 that live and breed in the water, to which they 

 are conspicuously adapted. Their bodies are fish-like in 

 shape and hairless, save for a few sensory hairs on the 

 head ; but are protected by a thick layer of fat, known as 



Cetaoea. 



