176 ANIMALS 



repetition of these events causes another hitch forward, and so 

 the worm progresses by a process called inching. 



411. If the setae are set forward and the muscular contraction 

 repeated in the same sequence as before, the animal inches back- 

 ward, and practically with equal facility. This is the chief 

 method of locomotion of the earthworm, but nereis, by wave- 

 like contractions of the longitudinal muscles, alternately right 

 and left, causes a serpentine movement of the body by which 

 it creeps along. A similar mode of contraction, alternating 

 between the dorsal and ventral muscles, causes an undulatory 

 up and down motion by which nereis swims. The leech swims 

 in the same w^ay. 



412. In Arthropods, the method of locomotion is totally 

 different. Here the animal, both body and appendages, is 

 encased in a series of rings or cylinders composed of the stiff 

 cuticula and permitting of practically no change of form within 

 the individual segments. Howxver, the segments are flexibly 

 connected with each other and provided with muscles attached 

 in such a way that the segments may be moved with respect 

 to each other. Thus the ambulatory appendage of the cray- 

 fish consists of six segments, connected with each other and with 

 the body by six hinge-like joints. The muscles lie inside the 

 cylindrical segments of integument to which they are attached 

 and extend across the joint from segment to segment. When 

 a muscle contracts, it flexes the appendage at the joint between 

 the two points of attachment. The muscles are arranged in 

 pairs at each joint, and the two muscles of a pair move the ap- 

 pendage in opposite directions. The appendages operate 

 purely as levers, and locomotion is wholly due to leverage action. 

 This is true also of the action of the tail fin, which by its power- 

 ful strokes causes the body to shoot backward. The wings 

 of insects likewise operate as levers. In these cases the ful- 

 crum of the leverage action is the water or air instead of the 

 earth. 



