492 VERTEBRATE LIFE AND ORGANIZATION 



most marsupials the extraembryonic membranes, and chiefly the yolk 

 sac, simply absorb a "uterine milk" secreted by the mother. There is no 

 intimate union between the extraembryonic membranes and the uterine 

 lining as there is in most eutherians. 



Marsupials are born in what we would regard as a very premature 

 stage. Their front legs, however, are well developed at birth, and the 

 young jnill themselves into a marsupium, or pouch on the belly of 

 the mother, attach to a nipjjle, and there complete their development. 

 It has long been believed that they are too immature to suck, and that 

 milk is squirted from the mammary glands into their mouths. But in his 

 recent book on " Tossums," Hartman relegates this notion to the limbo 

 of false myths and cites careful experiments and observations proving 

 that the young do indeed suck. A forward extension of the tubular 

 epiglottis dorsal to the secondary palate completely separates the diges- 

 tive and respiratory tracts, and breathing and feeding can take place 

 concurrently. 



Marsupials were world-wide during the early Cenozoic, but as eu- 

 therians began to spread out, marsupials became restricted. They have 

 been most successful in those parts of the world where they have been 

 isolated from competition with eutherians. They are the dominant type 

 of mammal in Australia, and have undergone an adaptive radiation and 

 have become specialized for many modes of life. There are carnivorous 

 marsupials such as the Tasmanian wolf, ant-eating types, molelike types, 

 semiarboreal phalangers and koala bears (the original "Teddy-bear"), 

 plains-dwelling kangaroos and rabbit-like bandicoots. In contrast, the 

 only marsupial present in North America is the semiarboreal opossum. 



213. Adaptive Radiation of Eutherians 



/nsecf/vores. The eutherians, or placental mammals, as they are 

 frequently called, are the most successful mammals in all the parts of 

 the world that they have reached. They have radiated widely and 

 adapted to nearly every conceivable ecologic niche upon the land. Others 

 have rcadapted successfully to an aquatic mode of life, and some have 

 evolved true flight. 



The most primitive eutherians, that is, the stem group from which 

 the other lines of descent evolved, were rather generalized, semiarboreal, 

 insect-eating types of the order Insectivora. Modern shrews and moles 

 (Fig. 24.15) are specialized insectivores. 



Flying Mammals. Bats, order Chiroptera, are closely related to this 

 stem group, and are sometimes characterized as flying insectivores. As in 

 other flying vertebrates, the pectoral appendages have been transformed 

 into wings. Bat wings are structurally closer to those of pterosaurs than 

 to birds' wings, for the flying surface is a leathery membrane, but the 

 wing of a bat is supported by four elongated fingers (the second to fifth) 

 rather than by a single one as in the pterosaur. The wing membrane 

 attaches onto the hind legs, and in some bats the tail is included in the 

 membrane. The first finger is free of the wing, bears a small claw and 

 is used for grasping and clinging. The hind legs are small and are of 



