VERMES 169 



Excretory Organs. — The nephridia, kidney-like ex- 

 cretory organs, are responsible for the elimination of 

 nitrogenous waste. Each of them is a curiously twisted 

 tube Avhich opens to the body cavity through a funnel- 

 shaped end and empties to the exterior through the other 

 end (Fig. 96). 



The Nervous System. — The nervous system extends 

 the length of the body on the ventral surface. It is a cord 

 with a small swelling or ganglion in each segment, made 

 of two cords only partially joined. This is particularly 

 interesting because in some of the other worms the two 

 cords are entirely separate and lie on opposite sides of 

 the body. At the anterior end these cords separate and 

 pass in a ring around the alimentary canal. The junc- 

 ture on the dorsal side is marked by a double ganglion. 

 The serial arrangement of the ganglia in the ventral 

 cord is very important, for on it those people who trace 

 the ancestry of the vertebrates to the segmented worms 

 base their claim. In higher animals the spinal cord 

 retains this characteristic even when the body shows no 

 other trace of segmentation (Figs. 70, 71). 



Reproduction. — The arrangement by which the eggs 

 and sperm come in contact with each other is extremely 

 interesting. They can not be turned loose as in aquatic 

 forms, for the delicate eggs and sperm would die if they 

 were left unprotected on the ground. The openings 

 seen near the ninth and tenth segments lead into little 

 sacs called seminal receptacles situated two on each side. 

 In the thirteenth segment tiny ovaries which produce 

 the egg cells lie one on each side. These open to the 

 outside on the fourteenth segment. Near them are the 



