Metagenesis is observed in the context of ............... .

In which phylum are the animals characterized by cells arranged in two embryonic layers?

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

Which of the following statement$(s)$ is/are correct regarding phylum $Coelenterata$ (Cnidaria)?
$(A)$ They are aquatic,mostly marine,sessile or free-swimming,radially symmetrical animals.
$(B)$ They have a central gastro-vascular cavity with a single opening on hypostome.
$(C)$ Digestion is extracellular and intracellular.
$(D)$ Their members are $Sycon$,$Spongilla$ and $Euspongia$.

In the phylum $Coelenterata$ $(Cnidaria)$,what are the sequential names of the asexual and sexual stages?

Describe the location and function of the following structures in Phylum Coelenterata:
$(i)$ Tentacles
$(ii)$ Hypostome