MAMMALIA 449 



currents interchange materials by osmosis through the capillary 

 walls. There is no direct continuation of nervous tissue across 

 from the mother to the young. Notwithstanding this lack of 

 direct connection- each exerts a very profound effect upon the 

 other. It is known that the embryo secretes materials which 

 passing into the blood of the mother produce the growth of the 

 mammary glands that leads finally to lactation. Proper nutri- 

 tion, malnutrition, and poisons in the blood of the mother 

 greatly influence the growth and normality of the offspring. 



463. Classification of Mammals. In the introductory sur- 

 vey in 448 the three subclasses have already been outlined. 

 Subclass I. Ornitkodelphia (bird uterus), or Monotremata. 



FIG. 238. 



FIG. 238. Duck-bill (Ornithorhynchus anatinus) . Photographed by Folsom. 



Questions on the figure. What are the peculiarities of Ornithorhynchus? 

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



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 

 sometimes 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 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, 

 ag 



