CHAPTEE XI 

 VEEMES 



Irritability of the Earthworm. — The most widely 

 known representative of this group is the common earth- 

 worm. Every one is familiar with its elongated, symme- 

 trical, curiously-ringed surface. It is extremely irri- 

 table and when stimulated it moves readily and quickly. 

 It has no definite sense organs, but a tactile sense is 

 widely distributed over the surface of its body and it is 

 so sensitive that it responds even to sound vibrations. 



Mechanics of Locomotion. — Movement from place to 

 place is controlled mechanically by a definite arrange- 

 ment of muscles. The worm lifts its anterior end 

 and stretches, thereby becoming long and narrow. It 

 puts down this end and holds fast. It then lifts the pos- 

 terior end and by becoming short and thick draws it for- 

 ward. This end then holds fast and the anterior end 

 again stretches. Through repetition of this process the 

 animal progresses. Two sets of antagonistic muscles 

 control this alternate elongation and contraction. They 

 are arranged so that one set runs lengthwise through the 

 body while the other set encircles it. As one set con- 

 tracts the other set relaxes. When the longitudinal mus- 

 cles contract and the circular muscles relax, the worm 

 becomes short and thick ; when the circular muscles con- 



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