"Sea pen" and "sea fan" are marine.........

Which of the following organisms has no blood but still respires?

"Polymorphism" is observed in members of the class:

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 $Euspongia$.

Sea anemone belongs to which class?

Absence of circulatory system in $Hydra$ is compensated by