III.] THE EARTHWORM. 257 



undergoes a bending upon itself and a sudden in- 

 crease in calibre; substitute a high power and fix 

 your attention on this last-named area. Note 



a. The thick glandular walls of the tubule in which 

 ciliary action is going on (glandular segment). Care- 

 ful examination will reveal the presence of large 

 oval transparent nuclei. 



b. The thin-walled segment, a delicate transparent 

 tube lying immediately adjacent to the lesser limb 

 of a. It also is bent upon itself and ciliated. 



Its walls appear as mere hard outlines, and the 

 transverse diameter of its two limbs collectively is 

 about equal to that of the lesser one of a. 



c. The muscular segment. Examine under a low 

 power. The position in which this segment will 

 be found varies, as it is generally displaced in the 

 process of manipulation. Like both a and b it is 

 bent upon itself, and can be at once recognised by 

 its great transverse diameter as compared with the 

 rest of the organ. Follow it along it will be 

 found to become suddenly constricted at the point 

 at which the glandular segment enters it. 



Its aperture of communication with the exterior 

 is best seen in section. (Cf. Sect. H. i c.) 



d. Look for the internal loop (i a). Note that both 

 glandular and thin-walled segments enter into its 

 composition. 



e. Look for the nephrostome (i b\ Having found it, 

 examine under a high power. It is lined by a 

 columnar ciliated epithelium, the individual cells of 

 which are very large and clearly defined. Follow 



M. i 



