Planula

For the 'Planula hypothesis' of the ancestral animal, see urmetazoan.

A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species and also in some species of Ctenophores, which are not related to cnidarians at all. Some groups of Nemerteans also produce larvae that are very similar to the planula, which are called planuliform larva.^[1][2] In a few cnidarian clades, like Aplanulata and the parasitic Myxozoa, the planula larval stage has been lost.^[3][4]

Development

Planula stage of Clytia hemisphaerica

The planula forms either from the fertilized egg of a medusa, as is the case in scyphozoans and some hydrozoans, or from a polyp, as in the case of anthozoans.

Depending on the species, the planula either metamorphoses directly into a free-swimming, miniature version of the mobile adult form, or navigates through the water until it reaches a hard substrate (many may prefer specific substrates) where it anchors and grows into a polyp. The miniature-adult types include many open-ocean scyphozoans. The attaching types include all anthozoans with a planula stage, many coastal scyphozoans, and some hydrozoans.^[5]

Feeding and locomotion

The planulae of the subphylum Medusozoa have no mouth, and no digestive tract, and are unable to feed themselves (lecithotrophic), while those of Anthozoa show more variation and can be both lecithotrophic, parasitic or feed on plankton or detritus.^[5][6][7]

Planula larvae swim with the aboral end (the end opposite the mouth) in front.^[5][1]