Excretory System

            Annelids are considerably larger and more complex than flatworms.  In addition, they possess a well-developed circulatory system, which can function in conjunction with other systems in the body.  Increased size has made it impossible even for marine members of the group to rely on simple diffusion for the removal of nitrogenous wastes from their site of production.  Thus, the excretory system functions both in osmoregulation, and in the removal of nitrogenous wastes.  The excretory organs, nephridia, are capable of removing fluids and wastes from both tissue fluids and blood.  Each nephridium (there is usually a pair of nephridia in all but the most anterior segments) is a complex organ consisting of several parts (Figure 3).  A ciliated funnel, the nephrostome, opens to the coelom just anterior to the septum separating two segments.  Beating of the cilia drives fluids into the nephrostome from the coelom and then through a long tubule which passes through the septum into the next most posterior segment.  A network of blood capillaries surrounds the tubule and work in the exchange of materials, removal of wastes and re-absorption of water and other useful materials.  This exchange takes place between the tubule contents and the blood.  The tubule leads to an enlarged bladder and materials finally exit the body through a small nephridiopore.

            Examine the demonstration specimen of Lumbricus sp., which has been dissected to illustrate the position of the nephridia in each segment.  Use Figure 2 (above) to aid you in orienting and locating the nephridia.

 

Figure 3:  Diagram of One Nephridium of Lumbricus terrestris.

Nephridia