Which of the following is not a characteristic feature of cnidarians?

Select the taxon mentioned that represents both marine and fresh water species.

$S$ - 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 $Euspongia$.

Write the characteristics of Phylum Coelenterata (Cnidaria).

$A$ diffused nervous system is observed in . . . . . . .