Animated coloured scanning electron micrograph (SEM) of Nematocyst (ejected tubule) from a tentacle of the box jellyfish (Carybdea alata). This box jellyfish belongs to the Cubozoa group of Cnidarians. The larval stage of Cnidarians is called the planula and it is a free swimming developmental stage. Cubozoan planulae are pear-shaped, have pigment spots that may be sensitive to light, and swim for a few days using cilia. After a planula settles, it grows into a polyp. The cubozoan polyp can crawl around and bud off more polyps. Like all cnidarians, cubozoans possess nematocysts, cells that fire a barb and transfer venom. When a nematocyst contacts something that might be prey or predator, the barb uncoils and fires from the capsule along with toxic venom.