$A$ coral island with a central shallow lake is known as

In which animals is metagenesis observed?

Cnidoblasts are found in .........

$A$ coelenterate commonly known as 'fresh water polyp' is

$S$ - Statement: Some species of $Coelenterata$ exhibit polymorphism.
$R$ - Reason: In $Coelenterata$,the medusa form is observed as the asexual stage.

Hydra has