30 



ANIMAL LIFE 



of oars. The effort necessary to move the boat slowly 

 through calm water is but slight in comparison with 

 the shove and lift which we have to give in order to 

 bring the boat down to the water. On the water the 

 boat is relieved of its weight and becomes buoyant ; 

 on land the weight tells ; never, even in our dreams, do 

 we imagine a boat drifting along the land. 



The movements of animals are of these kinds. The 

 body is the boat, its muscles represent the man, and 

 its limbs replace the oars and boathook. Those animals 

 which creep over the ground use their legs as the boat- 

 man uses his punt-oar, pressed against the rock, sand, 

 or mud. Many animalcules, most worms, Crustacea, 

 insects, and the vertebrates except fishes, press off in 

 this way against some resistant medium. Burrowing 

 animals lay hold of the sides of their retreat with 

 hooks and claws as a boathook can be used to draw 

 craft along river banks — of such are worms, many 

 Crustacea, and insects. Swimming animals undulate 

 the muscles of their back and tail as an oarsman 

 twists a scull in the stern of a boat and lays hold of 

 new columns of water with the blade first in one 

 direction and then in another. In this way, bending 

 the body from side to side into S and 2 shaped curves, 

 the tail of the fish, the body of water-worms and of 

 aquatic larvae, grip a mass of water, and, using this 

 as a resistant mass, bring the power of their muscles 

 to bear upon it momentarily, then, instantly twisting 

 into an inverse curve, grip a new mass, and so gain a 

 new forward impetus. In a somewhat different way 



