25 S ELEMENTARY BIOLOGY. [CHAP. 



it back : it is continuous with the tliin-wallcd scii- 

 ment. 



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-wallcd, 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. 



f. The ncphridial hlood-plcxiis. 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 iiepJiridial vessels; two thin-walled 

 tubes running side by side, parallel w^ith tlie long 

 loop of the nephridium. They are connected 

 by an cxo'ctory plexus^ the smaller vessels of 

 which form a bold series of loops on the surface 

 of the organ. 



/5. The appendages of the excretory vessels; a series 

 of relatively large globular dilatations, filled with 

 blood and generally crowded with minute colour- 

 less non-nucleated corpuscles. 



