The Excretory System

  • Typically each nephridium consists of a ciliated funnel called nephrostome opening into the body cavity and leading into a long coiled tube which is thrown into loops, and opens to the exterior by the nephridiopore. The looped portion of the nephridia consists of three regions, first a wider ciliated portion, the second a glandular and non-ciliated portion; and the third a non-glandular and non-affiliated portion. This kind of typical nephridia is of common occurrence among oligochaetes, in Lumbricus, (European species) and in Drawida (Indian species). The nephridia that open to the exterior are called exonephric nephridia.
  • In some cases all the nephridia open into a longitudinal canal that discharge the waste into the posterior part of the gut and is called the enteronephric nephridia.
  • There are two types of nephridia found among oligochaetes, one smaller micro and other larger meganephridia.
  • The micronephridia are very small and much simpler, hang freely from the septum with their nephrostomes, the short narrow tube is the main part of the micronephridial tube, is an spirally twisted loop with a ciliated lumen. These are enteronephric nephridia.
  • Many earthworms have exonephric meganephridia. Taxonomists prefer to call holonephridia (one pair in each segment) and meronephridia (more than one pair in each segment). The holonephridia are always single and relatively a few in number in each segment. Meronephridia are usually very small and numerous in each segment.

Fig: View of various types of nephridia : A. large enteronephric; B. micromeronephridia; C. megameroephridia


Fig: View of stomate holonephridia in Perionyx sansibaricus