Nephrostome occurs in

Give a scientific reason: The blood vascular system of leeches is also called the haemocoelomic system.

The clitellum of $Pheretima$ $posthuma$ is primarily associated with which of the following?

How many types of nephridia are present in earthworm?

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.

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