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

Assertion $A$: Metameric segmentation is observed in Annelids.
Reason $R$: The body of an Annelid is divided into two equal halves by a single axis.
Which option is correct for Assertion $A$ and Reason $R$?

External segmentation is absent but internal segmentation is present in

Annelids have a central nerve cord that is

Select the correct option:
$(1)$ Annelids are the first animals to possess a circulatory system.
$(2)$ The heart is called myogenic.

Which of the following animals does $not$ undergo metamorphosis?