258 ELEMENTARY BIOLOGY. [CHAP. 



defined. Follow it back ; it is continuous with the 



thin-walled segment. 



The actual course taken by the coils of the seg- 

 mental organ can only be made out with very con- 

 siderable trouble. Sufficient is here described to 

 render it clear that three segments are present — an 

 internal thin-walled, a middle glandular and an 

 external muscular one respectively ("internal" and 

 "external" having reference to the fact that these 

 segments communicate respectively with the body- 

 cavity and the exterior). The only source of diffi- 

 culty which will be found in attempting to unravel 

 the whole, is the internal loop (d) formed by a 

 secondary folding of both thin-walled and glandular 

 segments. Eliminate that, and the whole resolves 

 itself into a tube of three segments — each bent upon 

 itself. Cilia are entirely absent for at least the 

 terminal third of the organ, and they elsewhere 

 occur only in isolated spinally disposed series. 



/ The nephridial blood-plexus. Remove, as directed 



above, the segmental organ of a worm which has 



been at least two days in alcohol. Examine in 



water under a high power. Its sheath carries a 



complicated series of blood-vessels, conspicuous 



by their yellow colour. Note — 



a. The main nephridial vessels; two thin-walled 



tubes running side by side, parallel with the long 



loop of the nephridium. They are connected 



by an excretory plexus, the smaller vessels of 



which form a bold series of loops on the surface 



of the organ. 



/?. The "blood glands" of the excretory vessels; a 



series of relatively large globular dilatations, 



filled with blood and generally crowded with 



minute colourless non-nucleated corpuscles 



