The poisonous fluid present in the nematocyst of $Hydra$ is

Match the following columns:

$a$. Pennatula	$1$. Brain coral
$b$. Meandrina	$2$. Sea-fan
$c$. Gorgonia	$3$. Sea-pen
$d$. Adamsia	$4$. Sea-anemone

Select the correct sequence for $a, b, c, d$.

The characteristic feature of Phylum $Cnidaria$ is .....

Pneumatophore helps in

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 $Euspongia$.

"Portuguese man-of-war" is a/an: