SUPPORT, MOTION, AND SENSATION 335 



mals must be able to go from one feeding area to another. This 

 holds good not only from the standpoint of competition for food but 

 also from that of avoiding unfavorable climatic conditions, such as 

 drought, which destroys those animals that are unable to keep on 

 moving to a better feeding ground. Other animals use this same 

 ability of movement in flight and so survive by being able to escape 

 capture. Lastly, the part played by motion in perpetuating species 

 should be mentioned. The strutting and bowing of a male pigeon, 

 or the battle between two male deer in the silence of the forest are 

 common examples of movement employed in the perpetuation of the 

 species. 



Protoplasmic Extensions 



The concept of movement is usually associated with the contraction 

 of muscles, but muscles do not tell the whole story. Three distinct 

 types of locomotor devices — namely, pseudopodia, flagella, and cilia, 

 which are so characteristic of the protozoa, have already been 

 described. 



The cirri of protozoa are probably the most highly specialized of all 

 unicellular motile structures as they may be moved in any direction. 

 Certain organisms like Stylonychia or Euplotes actually walk or run on 

 the tips of their cirri. The action of the cirri is thought by some to 

 be controlled by a so-called "neuromotor apparatus" present in these 

 "simple" one-celled organisms. 



Der mo-Muscular Sacs 



Many of the soft bodied invertebrates possess locomotor muscles 

 concentrated in the outer layers of the body. The earthworm is an 

 example of such a type. The body is shortened by the contraction 

 of the inner longitudinal muscles and elongated by the action of the 

 outer circular set lying immediately beneath the cuticula and hypo- 

 dermis. 



Water Vascular Systems 



The echinoderms have exclusi^'e patents on this method of loco- 

 motion that functions by means of water pressure in their numerous 

 tube-feet. The apparatus opens on the dorsal surface of a starfish, 

 for instance, through a sievelike structure, called the m.adreporite. Sea 

 water may be added to the so-called amhulacral fluid through the 



