Septal Nephridia

They are the largest nephridia and lie attached to both sides of each intersegmental septum behind the 15th segment.

Structure of Septal Nephridia

It consists of the following parts:

1. 
   Nephrostome: It is the ciliated funnel communicating with the coelom. It consists of an elliptical pore bounded by upper and lower lips. Upper lip is formed of large central cells and 8-9 marginal cells. Lower lip is formed by 4-5 compact cells and the cells are ciliated.
   


2. 
   Nephridium: Neck is a short narrow ciliated tube that joins nephridium to the body.
   


3. 
   Body Of Nephridium: Body consists of two parts: short straight lobe and a proximal lobe is connected with terminal duct via the neck.
   


4. 
   Terminal duct: It is a short narrow and ciliated tube leading into a septal excretory duct of its side.
   


5. 
   Nephridial tubule: It consists of glandular mass transversed by a coiled tube having four ciliated tracts in its course -1 in neck, 2 in body, and 1 in terminal duct. There are four parallel tubules in the straight lobe- 3 basal part, 2 in apical part, and 1 in neck and terminal duct.
   

Arrangement of Septal Nephridia

• Each septum behind the 15th segment bears 4 rows of septal nephridia – 2 on anterior pace and 2 on posterior face.




• Each row may contain 20-25 nephridia as there are 80-100 nephridia on each septum. 




• Nephridia remain suspended freely in the coelom of each segment and are attached only by their terminal ducts.




• Their terminal ducts open into a pair of septal excretory canals that run parallel to the commissural vessel of their own side.




• These canals discharge their contents dorsally into a pair of supra intestinal excretory ducts situated just above the intestine and beneath the dorsal blood vessel extending from 15th to last segment.




• These ducts open into the intestine in each segment through narrow ductules each having a sphincter opening. They are enteronephric nephridia.