INTERNAL ANATOMY OF THE EARTHWORM l (2) 



1. Fasten the specimen, which you worked on the last time, in a dis- 

 secting pan as before. Make a transverse cut through the alimentary 

 canal, a few segments posterior to the gizzard. Gently move the cut ends 

 slightly to one side and find the whitish ventral nerve cord lying 

 directly below in the median fine. Carefully remove the portion of the 

 alimentary canal lying anterior to the cut just made, as far as the pharynx, 

 but in so doing be sure first to cut all the septa which hold it, so as not 

 to disturb the underlying nerve cord. 



2. Trace the nerve cord to the anterior end of the pharynx, where it 

 divides to form a nerve collar (circumpharyngeal connectives) which 

 encircles the anterior end of the pharynx. Examine the dorsal surface of 

 the nerve collar under the dissecting microscope and note the bilobed 

 swelling (cererral ganglion) which constitutes the drain of the animal. 

 If the dissection has been carefully made, nerves can be seen extending 

 from the cerebral ganglion to the extreme anterior end of the body. 



3. Examine the exposed portion of the nerve cord with the dissecting 

 microscope and note that, in each segment, it expands to form a slight 

 enlargement (ganglion) from which two pairs of lateral nerves arise. 

 Anterior to the ganglion in each segment, find another smaller pair of 

 nerves. All these nerves can be traced into the near-by muscles of the 

 body wall. 



4. Continue your examination with the dissecting microscope and 

 note, in each segment, lying close to the body wall on either side of the 

 nerve cord a tiny, greatly coiled, tubular structure (nephridium) which 

 is an important excretory organ. Note also the glistening, white longi- 

 tudinal muscle tissue of the body wall. Remove one or more of the 

 nephridia with a pair of fine forceps. Place on a slide or in a watch 

 glass with water. Study under the microscope and see if you can deter- 

 mine the main structural features. 



5. Make an enlarged drawing of the anterior end of the Earthworm 

 to show the structures as observed. 



1 B. pp. 95-98. 



283 



