The enzyme $Hirudin$ is present in the saliva of which of the following?

$A$ student was given a specimen to identify on the basis of the characteristics given below.
$(i)$ They are metamerically segmented.
$(ii)$ Presence of closed circulatory system.
$(iii)$ They have circular and longitudinal muscles for locomotion.
Identify the specimen.

Segmentation in the body is first observed in which of the following:

Describe the characteristics of Phylum $Annelida$.

Assertion $A$: Annelids exhibit extracellular digestion.
Reason $R$: The digestive tract in annelids is complete.
Which option is correct for Assertion $A$ and Reason $R$?

Which of the following is $NOT$ a characteristic of the phylum $Annelida$?