EARTHWORM 07 



Each metanephridium is composed of two distinct parts lo- 

 cated in different somites. The conspicuous white masses, one 

 on each side of the nerve cord in each somite, are the nephridial 

 tubes. Under the hand lens only a part of each mass shows its 

 tubular nature. The tube leads forward to a transverse septum 

 through which it passes and extends as a minute knobshaped 

 ncphrostome into the segment next ahead. The other extremity 

 of the tube is attached to the body wall through which it opens 

 as the nephridiopore. The location of this nephridiopore is best 

 observed in the study of the cuticula under section IV. Note 

 demonstration of stained nephridium. Draw three somites show- 

 ing arrangement of nephridia as seen under hand lens. 



RESPIRATION 



In the earthworm there are no special organs for respiration. 

 The. moist conditions under which the animal lives and the deli- 

 cate structure of the body wall make it possible for the entire 

 body surface to function in the process of respiration. 



NERVOUS SYSTEM 



1. The nerve cord lies between the nephridia. Conspicuous 

 . thread-like structure, slightly enlarged in each somite. How 



many lateral branches in each somite, and where situated ? 



2. Notice the small, whitish, two-lobed supra-pharyngeal 

 (jonglion, or brain, at the anterior end of the pharynx and on its 

 dorsal side. 



3. Very carefully lift the posterior end of the pharnyx and 

 trace the nerve cord forward to where it divides and passes 

 around the pharnyx to the brain, forming the circum-pharyngeal 

 collar. Make a diagram of the nervous system. 



BODY WALL 



With the hand lens examine the cut surface of the body wall. 

 Locate the muscle layers. Note the inner end-s of the setae .pro- 

 truding through the body wall. 



