LOCOMOTION DUE TO CONTRACTILITY 297 



in certain cases this creeping movement is due to active 

 secretion. In one case which has been especially well 

 studied (the one-ceUed gregarines), it has been shown 

 that, out of the food supphed to it, the protoplasm of the 

 cell secretes a slimj'- substance, just as the cells of the 

 salivary glands produce saliva. This slimy substance is 

 forced out of the cell body and accumulates at the rear 

 of the animal. Here it sticks to the surface and hardens, 

 and thus by its accumulation and resistance forces the 

 body along slowly before it. 



In the higher forms of animals we find no cases of motion 

 due entirely to secretion. In the snails, however, the 

 body secretes a slimy mucus which forms a surface for 

 the action of the muscular foot and M'hich we see as a shiny 

 trail when it hardens. The common house fly also secretes 

 a sticky fluid on the bases of the foot, by means of which 

 it is enabled to walk over smooth surfaces. The web of 

 the spider over which it travels, and by means of which it 

 raises and lowers itself, is a secretion of certain glands of 

 the body. These cases are evidently not cases of motion 

 due to secretion alone, but simply instances of where 

 secretion aids other methods. Motion due to secretion 

 alone is confined to a few one-celled forms. 



Locomotion due to contractility. — Practically all of 

 the free-moving animal forms owe their ability to change 

 position to the power of contractility exercised by the 

 protoplasm of part or all of the cells of the body. We 

 can distinguish three kinds of locomotion due to the 

 contractility of protoplasm : 



(a) Amoeboid movements. 



(b) Ciliary movements. 



(c) Muscular movements. 



