I46 THE EVOLUTION OF MAN. 



urogenitatis). The urinary bladder in the Monotremes also 

 opens into the cloaca, and is separate from the two urinary 

 ducts (Fig. 327, vo) ; in all other Mammals the latter open 

 directly into the urinary bladder. The structure of the 

 milk-glands, by means of which all Mammals suckle their 

 new-born young for a time, is also quite peculiar in 

 the Cloacal Animals. In them the milk- gland has no 

 nipple which the young animal can suck ; there is only 

 a peculiar sieve-like place in the skin, perforated with 

 holes through which the milk passes out, and from which 

 the young animal has to lick it. For this reason they 

 have also been called Nipple-less Mammals (Arnasta). 

 Again, the brain of the Cloacal Animals has remained at a 

 much lower stage of development than that of any other 

 MammaL The fore-brain, or cerebrum, is so small that 

 it does not overhang the hind- brain, or cerebellum. In the 

 skeleton (Fig. 196), the structure of the shoulder girdle, as 

 well as of other parts, is remarkable, differing entirely from 

 the same part in other Mammals, and resembling rather 

 those of the lower Vertebrates, especially Reptiles and 

 Amphibians. Like the latter, the Cloacal Animals have a 

 well-developed coracoid bone (coracoideum), a strong bone 

 uniting the shoulder-blade with the breast-bone. In all 

 other Mammals the coracoid bone (as in Man) has degene- 

 rated, has coalesced with the shoulder-blade, and appears 

 only as an insignificant process of the latter. These and 

 many other less striking peculiarities prove beyond doubt 

 that the Cloacal Animals occupy the lowest rank among 

 Mammals, and represent a direct intermediate form between 

 the Protamnia and other Mammals. All these marked Am- 

 phibian characters must have been present in the parent 



