Planula

A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species. Some groups of Nemerteans too produce larvae that are very similar to the planula.

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 adult form (such as many open-ocean scyphozoans), or navigates through the water until it reaches a hard substrate (many may prefer specific substrates) where it anchors and grows into a polyp (including all anthozoans with a planula stage, many coastal scyphozoans, and some hydrozoans). Planulae of the subphylum Medusozoa have no mouth or digestive tract and are unable to feed themselves, while those of Anthozoa can feed. Planula larvae swim with the aboral end (the end away from the mouth) in front.

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