III.] THE EARTHWORM. 25 1 



c The orifices of the excretory or segmental organs, 

 too minute to be examined here. They are best 

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



B. The sets and exoskeleton. 



1. Remove one of the setae from a post-genital segment 

 with a small pair of forceps, and examine in water 

 under a low power. Note — ■ 



a. Its form, curved and needle-shaped, blunted inter- 

 nally and terminating in a point. 



b. Its structure. Examine under a high power. It 

 consists of a chitinous rod, a portion of the central 

 axis of which is soft and granular. 



The setae vary somewhat in shape and calibre in 

 different regions of the body, especially so in the 

 clitellar and genital segments. 



c. The exoskeleton, a delicate iridescent cuticle, in- 

 vesting the whole body. Take a freshly killed 

 worm and strip off its cuticle under water; in doing 

 so note that it passes in to line the alimentary 

 canal at both mouth and anus. Transfer a small 

 portion of it to a slide, mount in water and examine 

 under a high power. Note — 



u. It consists of a colourless transparent membrane, 

 traversed by delicate lines or stria crossing each 

 other at an angle of 45 °. 



f}. Examine the same under a low power and note 

 the cuticular sacs of the setce, finger-shaped pro- 

 longations of the cuticle which occur in pairs. 

 They embrace, during life, the middle of the setse. 



It not unfrequently happens that the setas are 

 dragged away with the cuticle or it with them, upon 



