An elongate or oval larval form of Zoanthinaria: The zooxanthellas are pelagic larva that have a ventral band of very long cilia.

Zooanthellas, the minute unicellular green alga, thrive inside the tissues of small vascular growths on the outside of the mucous membranes of the corals, and they provide nutrients and oxygen through photosynthesis, and, in addition, furnish the corals with color.