MAMMALIA. 



439 



the blood vessels of the mother and the embryo are not con- 

 tinuous. The blood of the embryo is developed in the same 

 manner as its other tissues and is not derived from the mother 

 directly. 



452. Classification of Mammals. In the introductory 

 survey in 437 the three subclasses have already been out- 

 lined. 



Subclass I. Ornithodelphia or Monotremata. Mammals 

 whose mammary glands have no nipples; they lay eggs with 

 abundant yolk, which are hatched outside the body, as in birds. 

 The alimentary canal ends in a cloaca. One ovary is some- 

 times incompletely developed as in birds, and the oviducts open 

 separately into the vagina. The duck-bill or duck-mole lives 

 in water, or burrows in the banks of streams or lakes. It is 



FIG. 235. 



FIG. 235. Duck-bill (Ornithorhynchus anatinus). Photographed by Folsom. 



Questions on the figure. What are the peculiarities of Ornithorhyn- 

 chus ? What does the structure of its feet indicate as to its habits? 



eighteen or twenty inches long and is covered with soft fur. 

 Its eggs are laid in its burrows. Echidna or the spiny ant- 

 eater, lives in rocky places and captures ants by means of its 

 slender, sticky tongue. They are confined to Australasia and 

 are interesting chiefly because of their likeness to the reptile? 

 and birds (Fig. 235). 



