These cells form gonads.

This is the characteristic of all cnidarians. So, in this respect polyps and medusae are similar.

Members of the class anthozoa form corals by secretion of polyps.

Cnidarians possess radial symmetry in which an organism can be divided into similar halves by any plane that contains the main axis.They are also diploblastic, i.e. body is derived from two germ layers.

Planula larva is found in cnidarians.

It has nerve cells but no brain.

In cnidarians, there is regular alternation of sexual and asexual phases that is called metagenesis.

Nematocysts are the exclusive cells of  cnidarians, which are multifunctional such as used for offence, defence, locomotion etc that are not present in the poriferans.

It belongs to phylum echinodermata.

It is the stored food in hydra.

This is correctly matched.

 It is a specialised chracteristic of cnidarians as nematocysts are not found in any other phylum.

These cells give rise to nematocysts.

The green alga present in the digestive cavity of hydra is zoochlorella, which is a symbiont.

Corals provide ideal habitat for various animals. Therefore, it is considered as the richest marine biome.