Mammalia : ( Classification 



727 



Pinnae may have degenerated in connection with acquisition of bur- 

 rowing and aquatic habits.) The jaws, especially the upper, are pro- 

 longed forward to form a broad dorsoventrally flattened beak covered 

 with smooth hairless skin — hence the animal's common name, "duck- 

 bill." The legs are very short and each has five digits. A. strong claw on 

 each digit and webs of skin between digits doubly adapt them for 

 burrowing and swimming. The brown fur is short and thick. Situated 

 ventrally about midway of the length of the body is a pair of hairless 

 and slightly depressed spots of skin. On the surface of each spot open 

 the several ducts of a cluster of tubular mammary glands. The young 

 animal merely licks off the milk as it exudes from the pores. 



The duckbill is semiaquatic. It burrows into the bank of a stream 

 or pond. The entrance to the burrow is under water, but the inner end, 

 several yards distant, is high and dry. It feeds on small aquatic 

 animals. In the absence of calcareous teeth (temporarily present in 

 the embryo), horny plates covering both the upper and the lower 

 jaw serve for crushing hard-shelled prey. 



Echidna, of about the same size as the duckbill, differs from it in 

 many respects (Fig. 533). The tail is a mere vestige. The "beak" is 

 narrow and sharp-pointed. Teeth are totally lacking. The digits, five 

 on each foot, are not webbed and the claws are very strongly developed. 

 Dorsally and laterally, the animal bristles with long stiff spines (modi- 

 fied hairs) resembling those of a porcupine. Between the spines are 

 ordinary coarse hairs, and ventrally there is merely coarse hair. As in 





Fig. 533. Echidna. (Courtesy, Vogt and Specht: "Die Saugetiere in Wort und 

 Bild," Munich, F. Bruckmann-Verlag.) 



