How many pairs of hearts are found in earthworms (in $pairs$)?

Which of the following respiratory pigments is present in the blood of $Pheretima$?

What is the function of nephridia?

Which of the following structures is found in the $7^{th}$ and $9^{th}$ segments of $Pheretima$?

Following are the statements about the prostomium of an earthworm.
$(a)$ It serves as a covering for the mouth.
$(b)$ It helps to open cracks in the soil into which it can crawl.
$(c)$ It is one of the sensory structures.
$(d)$ It is the first body segment.
Choose the correct answer from the options given below.

Chloragogen cells are involved in