162 



BIOLOGY 



exterior. These nephridia have as their primary function the 

 removing from the body of the waste products containing 

 nitrogen, related to urea. Their function is thus similar to 

 that of the kidneys of the higher animals, and indeed their 

 structure is not unlike the kidneys of some of the ver- 

 tebrates. 



The Coordinating or Nervous System. The nervous system 

 consists of a central system and a peripheral (Gr. peri = around 



Fia. 79. DIAGRAM SHOWING THE NERVOUS SYSTEM 



IN THE FRONT END OF THE BODY 



eg, cerebral ganglia; 

 com, commissures; 

 m, mouth; 



oe, oesophagus; 

 pr, prostomium; 

 v, ventral cord. 



(Shipley and MacBride.) 



+ pherein = to bear) system, the latter composed of a large 

 number of nerves passing from the central system into the 

 various regions of the body. 



The central system. 1. The cerebral ganglia. These are 

 two nerve knots or ganglia, sometimes called the brain, united 

 together and lying above the pharynx in the anterior part of 

 the body cavity; Fig. 79 eg. From them, extending down- 

 ward and backward, a pair of cords or commissures (Lat. 

 committere to join together), com, pass around the phar- 

 ynx and unite with each other below on the ventral side 



