CHAPTER V 

 MOTION AND LOCOMOTION 



Simple forms of movement. The ability to move is 

 a more or less general property of living matter, since 

 it is found in many plants and all animals. One of the 

 most primitive devices for motion we have already seen 

 in the ameba, which moves about by changes of form in 

 its soft elastic body. Many other one-celled plants and 

 animals are able to move by the lashing of hair-like 

 projections from their bodies. Animals which float in 

 water have the advantage of requiring but little effort 

 to produce motion, since their weight is supported by 

 the water. Some aquatic animals much higher in the 

 scale of development than the ameba, as, for example, 

 certain jelly fishes, are able to get about quite swiftly 

 by hair-like processes similar to those of one-celled 

 plants and animals. 



More complex forms. Land animals, on the con- 

 trary, must either drag their bodies along on the ground 

 or lift their weight and, therefore, as their bodies in- 

 crease in weight the means of producing motion be- 

 comes more and more complex. In many worms, mo- 

 tion is accomplished by a laborious process of short- 

 ening and lengthening the body. Others attain much 

 greater speed by their many little legs, which lift the 

 body from the ground and enable them to take definite 

 steps unhindered by its drag. As we get higher in the 

 animal scale, the legs are longer and are stiffened in 



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