66 FIRST LESSONS IN ZOOLOGY 



in water and air are of peculiar help in this matter. 

 Water currents may carry an animal for great distances 

 without any work on its part; while air currents make it 

 possible for birds to soar with little effort. Flight by the 

 vibration of wings, as in birds and insects, requires the 

 greatest expenditure of energy, since the pushes against 

 the thin air must be made cjuickly and with great force and 

 be rapidly repeated to be effective for support and loco- 

 motion. Man in making locomotive machines, railway 

 engines, automobiles, steamships, etc., has met the 

 same conditions as the animals ; but the difficulties of 

 aerial locomotion are so great that he has not yet been 

 successful in inventing a mechanism for it such as has 

 been developed in the birds. 



The simplest and what may be called the most imper- 

 fect mode of locomotion is shown by the simplest ani- 

 mals. These, the Protozoa (see Chapter IX), are ex- 

 tremal}' minute, and mostly live in water. A single drop 

 of stagnant water may contain hundreds of them. Among 

 these simplest animals we can readily find forms illus- 

 trating at least four different modes of motion; all, how- 

 ever, are really but different forms of the same action, that 

 is, the contracting or shortening and thickening of a part 

 or the whole of the animal. In the Amceba (see Chapter 

 IX) the body is composed of a minute bit of soft, jelly- 

 like living substance called protoplasm, without any spe- 

 cial parts at all. By the varying contracting and relaxing 

 of the soft body portions of it are protruded and with- 

 drawn again, a slow and imperfect locomotion being 

 accomplished by this means (fig. 35). In the most rapidly 

 moving Amoeba the motion is very slow, while in the slower 

 ones it would take some hours to proceed a single inch. 



Some kinds of Protozoa show «hat is called streaming 

 motion. This is seen in forms in which the central por- 

 tion of the body iirotoplasm will flow along for a while in 



