"Polymorphism" is observed in members of the class:

Which of the following phyla exhibits diploblastic organization?

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 $Euspongia$.

It is a common cavity that performs both digestive and circulatory functions.

Coelenterate with a skeleton of $CaCO_3$ is called as:

In which phylum are neurons present,but nerves are absent?