The medusa of the thimble jellyfish has straight sides with sixteen grooves, and a flat top. The coronal groove between the top and sides provides flexibility. The margin of the bell has sixteen lappets (folds), the niches between these bearing alternately rhopalia (sense organs) and short tentacles (eight of each). There is no marginal ring canal. Symbiotic zooxanthellae give the bell an overall orangish-brown colour and the translucent mesoglea has dark flecks. This jellyfish can reach a diameter of 16 mm (0. 6 in) and a height of 20 mm (0. 8 in).