浮浪幼虫是一种自由生活，长有纤毛，两侧对称的幼虫。在所有的情况下，浮浪幼虫都是来自腔肠动物水母体世代所产生的受精卵合子，如钵水母纲、部分水螅纲、和一部分花虫纲。依据物种的不同，浮浪幼虫要不是立刻转变成成体的缩小版(例如很多大洋中的钵水母纲物种)，不然就会一直顺水漂流直到抓住坚硬的表面，以长成固着生活的水螅体(包括所有有幼虫期的花虫纲，许多海岸边的钵水母纲和一部份的水螅纲生物)。此外，钵水母纲的浮浪幼虫没有口和消化系统，因此无法摄取营养。

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.