XVII. WORMS 



The Earthworm. — The earthworm belongs to a group of animals 

 called the annelids. They are so called because the body is made 

 up of a large number of rings or segments. (Lat. annulus, a ring.) 



Study a living worm in order to answer the following questions. 



Careful study of several worms will show that the number of segments 

 varies, the larger worms having more segments. Do the segments vary 

 greatly in width? In shape? 



Notice the color of the worm; is it like that of the ground in which it 

 hves? Do the dorsal and ventral surfaces differ in color? Can you ac- 



The earthworm. Note the swollen area, the clitellum. Photographed Viy Overton. 



count for this? The earthworm is an example of a bilaterally symmetrical 

 animal, that is, one in which the right side is a counterpart of the left side. 

 Compare with the starfish or sea anemone. Are the latter animals also 

 bilaterally as well as radially symmetrical ? 



The two ends of the worm differ somewhat in appearance, the anterior 

 end being pointed and the posterior rather flattened. Tests made with a 

 pencil or other pointed object will show which end is more sensitive to touch. 

 Which end usually goes first in crawling? 



Measure an earthworm when it is extended and compare with the same 

 worm contracted. Note the difference in length. This is accounted for 

 when we understand the method of locomotion. Under the skin are two 

 sets of muscles, an outer set which passes in a circular direction around the 

 body, and an inner set which runs the length of the body. The body is 

 lengthened by the contraction of the circular muscles. How might the 

 body of an earthworm be shortened ? 



Put your finger over the under surface of the worm. Notice the rough- 

 ness. Examine the surface with a hand lens. Four rows of very minute 



^ See Hunter and Valentine, Manual, page 131. 



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