478 ANIMAL LIFE-HISTORIES 



lation, though it may perhaps be used as an emergency exit 

 as well. The milk-glands are comparable to those of the Spiny 

 Ant- Eater, but there is no pouch, and the eggs are deposited 

 in a rough sort of nest, lodged within the burrow, and made up 

 of dry vegetation. Professor Gregg Wilson was the first natur- 

 alist to find this nest, and owed his success to the discovery 

 that when the mother Duck- Bill goes out she throws up a little 

 wall of soil between the nest and the rest of the burrow, an 

 obviously protective arrangement. Anyone who dug out a 

 burrow in ignorance of this fact would find that it appeared to 

 end abruptly, whereas a little further excavation would reveal 

 the nest. 



POUCHED MAMMALS (METATHERIA). Here again we are 

 concerned with but one order (Marsupialia], the members 

 of this also being limited to the Australian region, with the 

 exception of Opossums (Didelphyidcz] and a small mouse-like 

 creature (Ccenolestes), which are American. Here, as in all 

 Mammals except the egg-laying forms, the ova are minute and 

 develop internally, there being special arrangements compen- 

 sating for the absence of food-yolk. Young Marsupials are 

 born in an extremely immature condition, and in the majority 

 of cases are at once placed by the mother in her large pouch, 

 and attached to the long teats there present. In Kangaroos 

 the opening of the pouch is in front (fig. 997), obviously the 

 most convenient position for creatures so constantly assuming 

 an upright posture. In other cases it is at the back. A number 

 of forms, e.g. most of the Opossums, are entirely devoid of this 

 structure. 



The pouch of a Marsupial is supported by two " marsupial 

 bones" (see fig. 734, p. 190) attached to the pelvis, and probably 

 equivalent to similarly-placed " epipubic " elements, often present 

 in Reptiles and Amphibia (see vol. i, p. 241, fig. 149). These 

 bones are also present in the Duck- Mole and Spiny Ant- Eater. 

 It would, however, be a mistake to suppose that the pouch of 

 the last-named animal is equivalent to that of, say, a Kangaroo, 

 for this is only one of many instances where organs of different 

 nature perform the same function. 



So helpless is a newly-born Marsupial that it is at first 

 unable to suck properly, and the milk is squirted down its 

 throat by the contraction of a layer of muscle covering the milk- 



