136 WORMS. 



III. Nervous System. Remove the seminal vesicles. 

 This will entirely expose the nerve cord which lies -ex- 

 tended along the floor of the body cavity its entire 

 length. Observe the ganglion-like swellings in each seg- 

 ment. Observe the nerves given off from it to each seg- 

 ment. Trace it forward to the pharynx. Observe that 

 it forks, surrounds the pharynx, and unites again above in 

 the cephalic ganglion (or brain). Compare this arrange- 

 ment of the central nervous system with that already seen 

 in crustaceans and insects. 



IV. Organs of Excretion. Observe the segmented 

 organs (nephridia), little, tangled, thread-like bodies, 

 attached to the posterior side of each septum, one on 

 each side of the body in each segment. Each of these 

 organs opens to the exterior by a minute pore, not here- 

 tofore noticed, and not easily discovered. Each opens 

 internally at the end which floats free within the body 

 cavity, by a minute ciliated orifice (discoverable only by 

 careful microscopic examination). These are excretory 

 organs, and drain out waste and worn-out materials from 

 the body. 



V. The Body Wall. Spread out the body wall per- 

 fectly flat, and pin it so. Observe its muscular lining. 

 Note that the longitudinal, fibrous bands are discontinu- 

 ous, as such, along the lines occupied by the locomotor 

 setae. These setae are moved by small muscles of their 

 own, not the least in importance in the animal's mechan- 

 ism. Strip up some of the longitudinal muscles, and 

 observe the circular ones that lie beneath. Outside these 

 is a layer of epidermis. 



VI. The Body Cavity. Cut a clean transverse section 

 of another specimen which has been hardened in alcohol, 

 using a very sharp knife or scalpel, and examine the cut 

 end with a lens. Note that the body is made up of two 

 tubes, one within the other, the inner one, the diges- 



