vin THE FUNCTIONS OF THE BODY 141 



contracts. The whole nervous system is essentially a 

 connected series of such reflex-arcs, all intricately joined 

 up with one another. 



3. Muscular Activity.- -The common Amoebae of the 

 pond, irregularly shaped corpuscles of living matter, a 

 hundredth of an inch or so in diameter, move about on 

 a substratum in a remarkable and perplexing manner. 

 Contracting a portion of their substance and pushing out 

 a corresponding amount in another direction, they pull 

 themselves, or glide, or roll along ; but the procedure 

 is not yet well understood. A similar kind of amoeboid 

 movement is common in certain cells of the body all 

 through the animal kingdom. Threadworms and lance- 

 lets are amonor the few animals that have not wandering 



o o 



amoeboid phagocytes. White blood-corpuscles or leu- 

 cocytes are familiar illustrations of units that move in 

 the amoeboid fashion. It is said that cells at the attach- 

 ing base of the freshwater Hydra may protrude amoeboid 

 processes, and thus move the whole polyp along the 

 water- weed. 



Another kind of locomotion is due to the action of 

 cilia or flagella. Just as the amoeboid mode is charac- 

 teristic of the Rhizopod Protozoa, so the movement by 

 cilia or flagella is characteristic of the Infusorians. Cilia 

 are lashes of living matter which are alternately flexed 

 and straightened, like our arm bent at the elbow and 

 rapidly extended again ; flagella move in an undulating 

 fashion like snakes in the water. The Planarian worms, 

 the Nemertean worms, the Rotifers or wheel-animalcules, 

 may be mentioned as multicellular animals which de- 

 pend, in part at least, in adult life on the activity of 

 external cilia ; and a very large number of free-swimming 

 larvae among Invertebrates (e.g. in Echinoderms, worms, 

 molluscs) move by means of cilia. Even a newly hatched 

 tadpole is covered with cilia. In threadworms and 

 Arthropods cilia and flagella are practically absent, but 

 in almost all other animals they are in evidence in various 

 internal parts of the body, e.g. in the lining of our air- 

 passages. In some cases, e.g. in certain starfishes, there 

 are external cilia, no longer of use in locomotion, which 



