MAMMALIA 



351 



five-toed and webbed, the webbing on the fore feet extending well 

 beyond the tips of the toes, but that of the hind feet being about as 

 it is in a water bird. Both feet are ai-med with sharp claws. The 

 beak is very wide and flat and is covered with soft, naked skin that 

 flares out at the base into sensitive flaps; this beak covering is highly 

 sensitive owing to the abundance of sense organs that are scattered 

 over its surface. There are no teeth in the adult, but instead, broad, 

 horny plates line the inside of the bill; these are used for crushing the 

 shells of bivalves and water snails, which constitute its chief food. 

 The young platypus has a set of milk teeth, all molars and eight or 

 ten in number; these are gradually worn off and then replaced by 

 plates. The eyes are small and beady; there is no external ear lobe; 

 the male has a spur on the heel like that of 

 the Echidnidge, but larger in size. The tail 

 is large and dorso-ventrally flattened; it is 

 used as a rudder in swinuning. 



The brain of Ornithorhynchus (Fig. 183) is 

 the most primitive brain known for a living 

 mammal. It is comparatively quite small, 

 and the cerebral hemispheres are smooth 

 and, like a reptile brain, entirely lacking 

 in convolutions. The habits of this creature 

 are purely aquatic, not unlike those of a 

 muskrat. It lives in stagnant, weedy ponds 

 or streams, feeding chiefly on moUusks, 

 crustaceans, and worms that are secured by 

 scooping up the muddy bottom with the 

 spoon-like snout. Provender is stored in 

 capacious cheek-pockets and is carried in 

 this way to the burrow, where it is eaten at 

 leisure. The burrow is dug deep into the bank of the stream, begin- 

 ning below the water-line and sloping upward until at a distance of 

 twenty-five to fifty feet it terminates in a large, dry chamber with 

 top ventilation. The chamber is comfortably lined with reeds and 

 rushes. 



Breeding Habits. — The eggs to the number of two or three are 

 laid in a nest of grasses, quite like a simple bird's nest. They are 

 somewhat smaller than those of Echidna and have a rather hard, 

 flexible shell, yellowish-white in color. They are incubated while 



Fig. 183. — Brain of 

 Orniihorynchus, dorsal 

 view, natural size; chl, cer- 

 ebellum; olf, olfactory 

 bulbs. Note lack of cere- 

 bral convolutions. (From 

 Parlver and Haswell.) 



