Which of the following is a characteristic of the dorsal blood vessel in an earthworm?

The region of the earthworm which is known as the 'forest of nephridia' is:

Earthworms are

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.

Define the following terms:
$(i)$ Peristomium
$(ii)$ Clitellum

In earthworms,which of the following statements is true regarding their blood?