ruYi.rM A.wi'ji./DA 87 



A STUDY OF THE EARTHWORM 



The above study of Nereis presented information concerning 

 what may he regarded as the typical external structure of marine 

 worms. Tlu're is another great group of worms typified hy the 

 earthworm. The worms of this group are terrestrial, though 

 some are aquatic and a few are equally at home in either kind of 

 environment. These animals feed mostly at night by crawling 

 entirely or partially from their burrows to the surface of the 

 ground. Their subterranean habits are associated with the 

 absence of certain structures which are possessed by the marine 

 worms. 



The structural features to which attention is called in the 

 following paragraphs apply to the large species, Lumbrirus 

 terrestris. If other species are studied, allowance must be made 

 for variations in structural details such as number of metameres, 

 the location of apertures, and the relations of internal organs 

 to segments, etc. 



External Characters. — Study a preserved specimen in a dissect- 

 ing pan containing a little water. For the finest structures 

 employ either a dissecting microscope or the lowest-power 

 objective of a compound microscope. Distinguish anterior 

 and posterior ends, dorsal and ventral surfaces. Is the "head" 

 as well differentiated as in the case of Nereis? The head of 

 Nereis is to be regarded as more nearly a typical annelid head, 

 while that of the eartiiworm is simplified or better fitted to the 

 needs^of subterranean life. Do any obvious vestiges of the 

 parapodia persist in the earthworm? 



As in Nereis, the mouth is surrounded by the first complete 

 segment or penstomium and is overarched by a small lobe, the 

 prostomium. C'audad of the peristomium the similarity of the 

 body segments is quite noticeable, each being readily determined 

 externally by the grooves extending around the body. Deter- 

 mine the total number of metameres exclusive of the prostomium. 

 In the cephalic third of the body of mature worms certain seg- 

 ments are swollen forming about the body a thick band or girdle 

 called the cliteUum. This is a glandular region which secretes 

 the cocoon in which the eggs of the earthworms are deposited. 

 Counting from the anterior end and omitting the prostomium 

 determine the metameres occupied by the clitellum. 



Pass the fingers in both directions along the ventral part of 

 the sides of the body. The sensation produced is due to rows 



