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A planula is the free-swimming, flattened, cilium, bilaterally symmetric larva of various species and some species of Ctenophora, which are not closely related to cnidarians. Some groups of also produce larvae that are very similar to the planula, which are called planuliform larva. In a few cnidarian clades, like Aplanulata and the parasitic Myxozoa, the planula larval stage has been lost.
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 . The attaching types include all with a planula stage, many coastal , and some .
Planula larvae swim with the aboral end (the end opposite the mouth) in front.
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.
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