160 



HIGH SCHOOL ZOOLOGY. 



returns from the intestines is subjected to the action of the 

 hepatic cells while passing through the poital circulation, hut 

 the venous blood from the posterior extremities is returned 

 directly to the heart tlirough the posterior vena cava, the kidney 

 (which first attains in the mamitials its reniform shape) receiv- 

 ing its blood su])ply entirely through the renal arteries. 



17. Having now examined into the structure of one of the 

 typical mammals, or Eutheria, let us see in what respects the 

 Prototheria and Metatheria differ therefi'om. 



The Prototheria embrace only a single order — Monotremata — 

 represented by two well-known forms, the Duck-mole {Orni- 

 tliorliynclius 'paradoxus), and the Porcupine Ant-eater {Echidna 

 liystrix) of the Australian region (Figs. 106 and 107). Out- 

 wardly, and in their 

 habits, these creatures 

 differ very much from 

 each other, the Duck- 

 mole being an aquatic 

 animal, with soft un- 

 der - fur covered by 

 stiff over-lying bristly hairs, which prevent the wetting of the 

 fur, with webbed feet which adapt it for swimming, and witli 

 a homy, toothless bill like a duck's, evidently adapted to 



secure food in the same 

 way ; while, on the 

 other hand, the Porcu- 

 pine Ant-eater is pro- 

 vided with stout bur- 

 rowing feet, by the aid 

 of which it opens the 

 Fig. 107.— Porcupine Ant-eater. (Echidna hyitrix). A ants' nests, on the con- 

 tents of which it feeds, is protected by stout spines instead 

 of the bristly coat of the Duck-moles, has a sharp snout 



Fig. 106.— The Duck-billed Platypus. 

 (Omithorhynchus paradoxus). 



