GENERAL LIFE OF MAMMALS. 735 



There is some uncertainty as to the primitive form of the placenta, 

 but the fact that it is discoidal in Perameles seems to confirm Balfour's 

 view that this form must be placed lowest. 



The formation of the allantoic placenta in Perameles is in essentials 

 the same as in Eutherian Mammals, but in details there are some 

 striking differences. The most noteworthy of these is, perhaps, that the 

 cells of the uterine epithelium, instead of disappearing at an early stage, 

 as in Eutherian Mammals, proliferate greatly, lose their cell outlines, 

 and by the increase of the nuclei form what is known as a syncytium. 

 Later this syncytial layer becomes highly vascular, and forms the 

 maternal portion of the placenta, whereas, as already seen, in Eutheria 

 it is the uterine mucosa which forms the maternal part of the placenta. 

 Into the vascular syncytium the allantoic capillaries grow down, until 

 ultimately maternal and foetal vessels are separated merely by their 

 endothelial walls and a mere trace of syncytial protoplasm. The 

 connection between the yolk-sac wall and the uterus is effected in a 

 similar manner. 



GENERAL LIFE OF MAMMALS 



Most Mammals live on dry land. The bats, however, 

 have the power of flight, and various forms are able to 

 take long swooping leaps from tree to tree. Thus there are 

 " flying phalangers," such as Petaurus^ among Marsupials ; 

 ' flying squirrels," such as Pteromys, among Rodents ; 

 u flying lemurs " (Galeopithecus\ allied to Insectivores. Not 

 a few are aquatic, all the Cetaceans, the two Sirenians, and 

 the Pinniped Carnivores, such as seals and walruses ; while 

 water-voles, beavers, otters, polar bear, and many others 

 are also at home in the water. Burrowers are well repre- 

 sented by moles and rabbits; arboreal forms by squirrels 

 and monkeys. 



As to diet, man, many monkeys, the pigs, and many others, 

 may be called omnivorous ; kangaroos, hoofed animals, and 

 most rodents are herbivorous ; the Echidna, the ant-eaters, 

 hedgehogs and shrews, and most bats, are 'insectivorous ; 

 most of the Carnivora are carnivorous ; dolphins and seals 

 feed chiefly on fishes ; but in most cases the diet varies not 

 a little with the available food-supply. 



The struggle for existence among Mammals is sometimes 

 keen among fellows of the same kind ; thus the brown rat 

 (Mus decumanus) tends to drive away the black rat (M. 

 rattus) j but stress, due to over-population, is sometimes 

 mitigated by migration, as in the case of the lemmings. The 

 struggle seems to be keener between foes of different kinds. 



