296 FIRST YEAR COURSE IN GENERAL SCIENCE 



a bird. The other pair corresponds to the legs of man and to 

 the hind limbs of the other animals mentioned. The single 

 fins on the upper and lower parts of the body of the fish are 

 useful for balancing and for steering. 

 In the nervous system and in the muscles, animals have 



A B 



FIG. 156. ORGANS OF LOCOMOTION 



Figure A represents the skeleton of a fin of a fish; B that of the fore 

 leg of a frog; C that of the wing of a bird. 



special organs which make evident use of the properties of 

 irritability and spontaneous motion. These properties are 

 not less necessary to the plant, but they are not so easy to 

 observe in plants as in the higher animals. Responses to 

 external influence on the part of animals are more imme- 

 diate than in the case of plants, and the motion is more 

 rapid and greater. Very few land plants move from place 

 to place; most land animals do so. In the case of animals, 

 there may be not only spontaneous motion of protoplasm 

 within the cell wall, and of the organs of the body, but also 

 locomotion, that is, motion of the whole body from one 

 place to another. 



