206 STUDIES IN PHYSIOLOGY 



is covered with a multitude of tiny hairlike projections which 

 are called ciVi-a, and these wave back and forth like little 

 oars, driving the animal through the water. In paramecium, 

 then, the outer surface only is adapted by the cilia for loco- 

 motion, whereas all the cell protoplasm of amoeba may be 

 concerned in the process. 



Earthworm. If one watches the locomotion of an earth- 

 worm, one sees that the animal first pushes forward the ante- 

 rior end and then draws up the posterior part of the body. 

 These movements are accomplished in the following way. 

 The outer wall of each joint or segment is composed of (1) 

 muscle fibers that run around the body (circular muscles), 

 and (2) those that extend from the anterior to the posterior 

 end of the segment (longitudinal muscles). When the circu- 

 lar muscles at the anterior end contract, the segments 

 become smaller in diameter, but longer. If now the longi- 

 tudinal muscles act, the joint is shortened, and in this way 

 the segments behind it are pulled forward. In the same 

 way the circular and longitudinal muscles in each of the 

 hundred joints, more or less, contract successively, and thus 

 a wavelike movement passes from the anterior to the pos- 

 terior end of the body. 



The forward movement of the worm would be impossible, 

 however, if there were not some means of anchoring the 

 segments after they have been pushed forward. Let one 

 grasp the tail end of a worm and draw the rest of the body 

 across the finger tip. The scratching sensation is caused 

 by four double rows of tiny bristles. These rows extend 

 the whole length of the animal on its ventral surface. To 

 the inner end of each bristle are attached small muscles by 

 which it can be pointed either forward or backward. The 

 bristles, therefore, not only make the ventral surface rough r 

 but also serve as very simple appendages to assist the 

 longitudinal and circular muscles in locomotion. 



Locomotion in Water. Most vertebrates that live in the 

 water are provided with appendages that act like paddles. 



