In which of the following classes of Annelida are one pair of ovaries and several pairs of testes found?

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

Closed blood vascular system,presence of hemoglobin in the blood,and chitinous setae or parapodia are the characteristics of which phylum?

$A$: Annelids exhibit extracellular digestion.
$R$: The digestive tract in annelids is complete.

$A$ 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.

In which of the following is true metamerism first observed?