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. 



f. 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 



