LAND AND WATER BOTTOM LIFE 309 



powerful, and the breastbone is keeled to permit their 

 attachment to it. The speed of some birds in flying is 

 extremely great, a hawk being able to fly at the rate of 

 one hundred and fifty miles an hour. 



The only mammal that flies is the bat, and here the 

 wings are formed by expansions of skin stretched over 

 the arms and legs as frames, while the fingers are enor- 

 mously developed for this framework. (See Fig. 141.) 



Organs of locomotion for land and water-bottom life. — 

 We find many curious adaptations in the animals which 

 move on land or over the bottom of the sea or ponds. 



The earthworm progresses by first contracting the cir- 

 cular muscles and thus extending the body. Then, as 

 the longitudinal muscles are contracted, the bristles on 

 the under side of the body are inserted into the surface 

 over which it is creeping;- as a result, the shortening of 

 the body produced by the muscle contraction causes the 

 body to be dragged forward. 



The leeches use the same muscles, but fasten the ends 

 of the body by means of suckers, so that their motion is a 

 progression of loops. 



Starfishes use their tube feet in a similar manner, first 

 by lengthening the foot and attaching it by the sucking 

 disk at the end, and then dragging the whole body for- 

 ward by contraction. 



In the mollusks, motion is produced by a creeping disk 

 or foot which in the clam ploughs through the sand and 

 forces the sharp-edged shell forward, while in the snail the 

 muscles contract and expand in such a way as to wrinkle 

 the bottom of the foot and thus push the body forward. 



Many land animals and sea-bottom animals have jointed 

 legs of some kind, operated by powerful muscles, usually 



