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MANUAL OF ZOOLOGY. 



for such an arrangement. As the young are born in such 

 an imperfect state of development, special arrangements are 

 required to secure their existence. When born, they are 

 therefore, in the great majority of cases, transferred by the 

 mother to a peculiar pouch formed by a folding of the integu- 

 ment of the abdomen. This pouch is known as the " marsu- 

 pium," and gives the name to the order. Within the marsu- 

 pium are contained the nipples, which are of great length. 

 Being for some time after their birth extremely feeble, and 

 unable to perform the act of suction, the young within the 

 pouch are nourished involuntarily, the mammary glands being 

 provided with special muscles which force the milk into the 

 mouths of the young. At a later stage the young can suckle 

 by their own exertions, and they leave the pouch and return 

 to it at will. In a few forms there is no complete marsupium 

 as above described ; but the structure of the nipples is the 

 same, and the young are carried about by the mother, adher- 

 ing to the lengthy teats. 



.The so-called "marsupial bones" (fig. 237) doubtless serve 

 to support the marsupial pouch and its contained young, but 

 this cannot be their sole function, 

 since they occur in the Monotremes, 

 in which there is no pouch. It is 

 believed by Owen that the function 

 of the marsupial bones is to assist in 

 the action of the mammae and testes, 

 serving respectively as a fulcrum for 

 the muscle spread over the mam- 

 mary gland and for the cremaster. 



The oviducts open into vaginal 

 tubes which open into a urogenital 

 canal ; but this does not open into 

 a " cloaca," though embraced by a 

 sphincter muscle common to it and 

 to the rectum. In other words, the 

 vagina is separated wholly or in great 

 part into two distinct tubes. The 

 testes are not abdominal throughout 

 life as in the Monotremes, but are 

 lodged in a scrotum. This, how- 

 ever, is placed in front of the penis, 

 and not beneath the pubic arch as 

 in most Mammals. From this un- 

 usual position of the scrotum, it is regarded by Owen as being 

 the same structure as the marsupial pouch of the female, 



Fig. 237. One side of the pelvis 

 of a Kangaroo, showing the 

 " marsupial bones " (nt) after 

 Owen. 



