The Collaborative International Dictionary
Planula \Plan"u*la\, n.; pl. Planul[ae]. [L., a little plane.]
(Biol.) In embryonic development, a vesicle filled with fluid, formed from the morula by the divergence of its cells in such a manner as to give rise to a central space, around which the cells arrange themselves as an envelope; an embryonic form intermediate between the morula and gastrula. Sometimes used as synonymous with gastrula.
(Zo["o]l.) The very young, free-swimming larva of the c[oe]lenterates. It usually has a flattened oval or oblong form, and is entirely covered with cilia.
Wiktionary
n. 1 (context biology English) In embryonic development, a vesicle filled with fluid, formed from the morula by the divergence of its cells in such a manner as to give rise to a central space, around which the cells arrange themselves as an envelope; an embryonic form intermediate between the morula and gastrula. 2 (context zoology English) The larva of a hydrozoan, which is free-swimming and covered in cilium.
WordNet
n. the flat ciliated free-swimming larva of hydrozoan coelenterates
[also: planulae (pl)]
Wikipedia
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.