THE ALLIES OF THE SQUIRREL 



413 



special characteristics. The name "Monotrem'ata" is given 

 to them in reference to the fact that these mammals possess 

 but a single opening (cloaca) through which the contents of 

 the intestine and the urinary and reproductive products pass 

 outward, as in the case of amphibians, birds, and reptiles. 

 Monotremes also stand alone among mammals in the fact 



Fig. 216. Duckbill 1 



that they lay eggs inclosed in a white, flexible shell, much 

 like the egg of a reptile. The duckbill gets its name from its 

 peculiar duck-like beak, which is toothless when the animal 

 is full grown, the teeth being shed after they have been used 

 for a while. The male has a hollow spur on the heel, which 

 is connected with a poison gland in the thigh. The duckbill 

 is semiaquatic, and lives in burrows in the banks of ponds 

 and streams. Its food consists of mollusks, small insects, 



1 From Lydekker 's Geographical History of Mammals. 



