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 class of animals possesses a closed circulatory system?

$A$ suctorial mouth is present in

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

In which of the following animals is hemoglobin dissolved in the blood plasma?