Why clitellum is called as forest of nepridia what function it does?

Dear student 

Clitellar segments are called ‘forest of nephridia’ in earthworm because it contains numerous number of nephridia in each segment (about 2000 to 2500).
 it is present in clitellar segments more clearly it is integumentary nephridia & it is attached to the body wall from 3 to last segment that open on the body surface i.e, it is exonephric