ANNULATA. 



197 



whether the earthworm knows the difference between light and 

 darkness. Which does it prefer? Has it any eyes? Test with a 

 bristle to see whether it is sensitive to touch. Is it equally so at 

 all parts of the body? Can it recognize moist soil as compared with 

 dry? Test. Of what value would such a power be in life? Can it 



d. m. 



FIG. 56. Longitudinal section of anterior end of Dero. A, sagittal section; 

 frontal section, showing anterior part of nervous system, b, brain; co, nervous collar 

 about the mouth; c.v., contractile blood vessels ("hearts"); d, dissepiment; d.m., 

 dermo-muscular wall; d.v., dorsal blood vessel; m, mouth; n.c., nerve cells; n.f., nerve 

 fibres; np, nephridia; p, prostomium; ph, pharynx; s, setae; sn, segmental nerves; v.g., 

 ventral chain of ganglia; v.v., ventral blood vessel. Only a part of the nervous system 

 and of the blood vessels belonging to the figure is shown. 



Questions on the Figure. Compare this with a cross-section of 

 Dero (Fig. 57) and identify the parts. How do the four anterior 

 segments differ from the others figured? Does the nervous system 

 continue the whole length of an animal like this? What organs 

 continue through all the segments? Which are repeated in prac- 

 tically all the segments? Which are confined to a few segments? 

 How do the segments in Dero compare with those of the tape- 

 worm? 



taste or smell? Can you test in such a way as to be sure of the 

 difference ? 



The nervous system may be studied from figures in the text- 

 books. See Figs. 56 and 57. There is a nerve-center (brain) just 

 above the mouth. This is connected with a chain of nerve centers, 

 on the floor of the body cavity, by a loop of nerve fibres. Every 



