Why are nephridia in earthworms,which are basically similar in structure,classified into three types? Mention the names of each.

(N/A) Although nephridia in earthworms are structurally similar,they are classified into three types based on their location and the segments in which they are found. These types are:
$1$. Septal nephridia: These are present on both sides of the intersegmental septa of segment $15$ to the last,which open into the intestine.
$2$. Integumentary nephridia: These are attached to the lining of the body wall of segment $3$ to the last,which open on the body surface.
$3$. Pharyngeal nephridia: These are present as three paired tufts in the $4th$,$5th$,and $6th$ segments.