22 



THE GROWTH OF ANIMALS 



Organs of animals. —The body of an ox is made up ot 

 many different parts, or organs, and eaclr organ lias a certain 

 function, or work, to perform. For example, the lungs per- 

 form the function of purifying the blood, the legs are for 

 locomotion, the ears for hearing, and the eyes for seeing. 

 We may define an organ, then, as a part of an animal that 

 has some definite function to perform. 



> The relation of function to structure. — The leg of a duck 

 performs the double function of walking and swimming. 



Fig. 8. — Foot of duck, blue jay, mole, and seal, respectively, showing 

 adaptation to fimction. 



It is accordingly provided with joints, muscles, toes, and a 

 web between the toes. The feet of a blue jay are fitted for 

 perching. The limbs of a seal are fashioneil for swimming 

 and the feet of a mole for digging (Fig. 8). The teeth of a 

 dog are constructed for tearing flesh, b;it the teeth of an 

 elephant are suited to grinding vegetal^le material. From 

 these examples, it will be seen that the structtu'e of an or- 

 gan is suited to the work, or function, it has to perform. 

 The higher we go in the scale of animal life, the more com- 

 plex becomes the function and structure of organs. This 

 is shown by the fact that a bird's wings are much more 



