STRUCTURE OF THE VERTEBRATES 77 



The monotremes lay small, reptile-like eggs which are hatched 

 in a nest built by the parents. The young are very immature at 

 the time of hatching, and are kept in the nest for some time. 

 The brain is better developed than that of any reptile, but very 

 primitive compared with other mammals. The body tempera- 

 ture is not constant, varying widely with the environment. The 

 skeleton is very reptilian in character, the legs being held later- 

 ally at right angles to the body. These characters have given 

 rise to the theory that they arose from the reptilian stem as a 

 separate line of evolution, and that they are not as closely re- 

 lated to the other mammals as are the Therapsida. The mono- 

 tremes have been described as a * 'museum of reptilian charac- 

 teristics". 



Ornithorhynchus, or the Duck-billed Platypus, is the best 

 known, although rarely seen in captivity, and it has never been 

 successfully kept in zoos in America or Europe. It is a water- 

 living mammal, inhabiting the ponds and quiet streams of Xew 

 Zealand. The nests are dug into the bank above the water level, 

 the opening being protected by the water which fills the tunnel. 



The more widely spread Echidnas live in drier regions and, 

 due to their spiny covering, are erroneously called hedge-hogs. 

 They are also called spiny ant -caters, and though many do live 

 on ants, the term is misleading for the name was long before 

 given to an entirely different group. Specimens have been kept 

 for long periods in zoological gardens. 



Sub-class Metatheria. One order, the Marsupialia, comprises 

 the entire sub-class. INIarsupial refers to the marsupium or 

 breeding pouch, which is simply a deep fold of skin on the lower 

 abdomen. The teats are within the pouch, and immediately after 

 birth the immature young climb into the pouch and remain 

 tliere until able to maintain independent existence. 



AVith the exception of the opossums of the American conti- 

 nent, and obscure South American forms, the order is limited 

 to Australia and the nearby territory. The American opossum 

 is perhaps the most generalized mammal in existence, its skele- 

 ton having remained practically unchanged since the early his- 

 tory of the race. It has survived in competition with the higher 

 mammals by remaining in hiding except at night. For the causes 



