What does the part labeled "a" in the given figure represent?

What is the function of the typhlosole in an earthworm?

The figure below shows the circulatory system of an earthworm. What are $P, Q$,and $R$?

In earthworms,the clitellum is located in which segments?

Earthworms are:

$A$ transverse section of $Pheretima$ taken through the $10^{th}$ segment is observed under a microscope. Which of the following structures can be observed in the section?