A planula is the free-swimming, flattened, bilaterally symmetric larval form of various cnidarian species & also in some rank of Ctenophores. Some groups of Nemerteans also create larvae that are very similar to the planula, which are called planuliform larva.
The planula forms either from the fertilized egg of a medusa, as is the effect in scyphozoans as living as some hydrozoans, or from a polyp, as in the effect of anthozoans.
Depending on the species, the planula either metamorphoses directly into a free-swimming, miniature version of the mobile person form, or navigates through the water until it reaches a hard substrate many may prefer particular substrates where it anchors as well as grows into a polyp. The miniature-adult classification include numerous open-ocean scyphozoans. The attaching types include any anthozoans with a planula stage, many coastal scyphozoans, and some hydrozoans.