The flow of blood in the dorsal blood vessel of an earthworm is:

The most important use of $Pheretima \text{ } posthuma$ is:

Study the following in $Pheretima$.
$I.$ Dorso-intestinal blood vessels
$II.$ External intestinal plexus
$III.$ Internal intestinal plexus
$IV.$ Ventro-intestinal blood vessel
Arrange the blood vessels in the correct sequence of blood flow from the ventral blood vessel to the dorsal blood vessel.
The correct sequence is:

Why is the earthworm called the friend of the farmer?

Hearts of Pheretima are situated in the segments

Which structure in $Pheretima\ \ posthuma$ functions similarly to the liver of vertebrates?