BACKBONED ANIMALS WHICH BREATHE IN AIR 431 



dition. They are placed by the mother in her pouch and affixed 

 to the long teats there situated. As for some time they cannot 

 even suck properly, the milk is forced into their mouths by the 

 contraction of a sheet of muscle which covers the milk-glands, 

 a procedure which would be pretty sure to cause suffocation were 

 there not some mechanical device to prevent it. 



