A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species and some species of ctenophores, which are not closely related to cnidarians. Some groups of nemerteans 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.