2l6 



COLLEGE ZOOLOGY 



External Features. The body of Lumbricus is cylindroid, 

 and varies in length from about six inches to a foot. The seg- 

 ments, of which there are over one hundred, are easily determined 

 externally because of the grooves extending around the body. 



dors.v 



fypk 



nepk 



hep 



FIG. 153. Transverse section through the middle region of the body of 

 the earthworm, Lumbricus. circ.mus, circular muscle fibers; cod, coelom; 

 dors.v, dorsal vessel; epid, epidermis; ext.neph, nephridiopore; hep, chloro- 

 gogen cells; long.mus, longitudinal muscles; neph, nephridium; nephrost, nephro- 

 stome; n.co, nerve-cord; set, setae; sub.n.vess, subneural vessel; typh, typhlo- 

 sole; vent.v, ventral vessel. (From Parker and Haswell, after Marshall and 

 Hurst.) 



At the anterior end a fleshy lobe, the prostomium (Fig. 156, i), 

 projects over the mouth (5); this is not considered a true seg- 

 ment. It is customary to number the segments with roman 

 numerals, beginning at the anterior end, since both external and 

 internal structures bear a constant relation to them. Segments 



