Through which structure does the coelenteron open at one end?

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 $Euspongia$.

What is the function of $Cnidoblast$?

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

$Portuguese\; man\; of\; war$ is

Define/Explain: Metagenesis.