WORMS 



209 



bristles called sdce may be found. Determine if these rows are single or 

 double. How many setse to a segment? 



Every segment except the first three and the last is provided with set«. 

 Each seta has attached to it small muscles, which turn the seta so it may 

 point in the opposite direction from which the worm 

 is moving. If you watch your specimen carefully, you 

 will see that locomotion is accomplished by the thrusting 

 forward of the anterior end ; then a wave of muscular 

 contraction passes dov\'n the body, thus shortening the 

 body by drawing up the posterior end. The seta? at the 

 anterior end serve as anchors which prevent the body 

 from slipping backward as the posterior end is drawn up. 



Make a drawing of several segments to show the ar- 

 rangement of setse. 



Notice that living earthworms tend to collect along 

 the sides of a dish or in the corners. This seems to 

 be due to an instinct which leads them to inhabit holes 

 in the ground. 



Test a worm by placing half in and half out of a darkened box. Which 

 does it seem to prefer, light or darkness? There are no eyes visible. A 

 careful study of the worm with the microscope, however, has revealed the 

 fact that scattered through the skin of the anterior segments are many little 

 structures which not only distinguish between light and darkness, but also 

 light of low and high intensity, as well as the direction from which it comes. 

 A worm has no ears or special organs of feeling. We know that although 

 a worm responds only to vibration's of low pitch, the sense of touch is well 

 developed in all parts of the bod,y. Notice especially how the worm uses 

 the whole anterior end to feel with. Jar the dish in which the worm rests 

 hghtly, and note the reaction that takes place. 



Feeding Habits. — Worms may be kept in the laboratory for 

 some time in a glass dish or box filled with soil. They feed on 

 pieces of lettuce or cabbage leaf. A feeding worm will show the 



Diagram to show how 

 movement of a seta 

 is accomplished; 

 M, muscles; ,S', seta; 

 W, body wall. 

 (After Sedgwick and 

 Wilson.) 



Forepart of an earthworm with the left body wall removed; a, dorsal blood vcssri; b, brain; 

 c, crop; g, gizzard; i, intestine; k, nephridia; m, mouth; n, one of the ganglia of the nerve 

 cord; oe, esophagus; p, pharynx; v, ventral blood vessel. Davison, Zoology. 



proboscis, an extension of the upper lip which is used to push 

 food into the mouth. The earthworm is not provided with hard 

 jaws or teeth. Yet it literally eats its way through the hardest 



hunter's BIOL. — 14 



