Which one of the following exhibits a concentric "tube within a tube" body plan?

Given below are two statements. One is labelled as Assertion $(A)$ and other is labelled as its Reason $(R)$.
Assertion $(A) :$ In $Nereis$,longitudinal and circular muscles help in locomotion.
Reason $(R) :$ $Nereis$ is an aquatic annelid which is monoecious and possesses lateral appendages called parapodia,which help in swimming.

Which one of the following is not a characteristic of Phylum $Annelida$?

$Aphrodite$,commonly known as sea mouse,is a:

Which one of the following is not a hermaphrodite animal?

Trochophore larva is found in