A jellyfish balance organ, usually consisting of a calcium or magnesium carbonate crystal, the movement of which against surrounding cilia enables the medusa to determine its position in the water.
The specialised structures present in the sensory niches between the four pedalia of cubozoan (box) jellyfish. It houses the ocellus (eye) and statocyst (balance organ). Rhopalia are also present, although less obvious, in scyphozoan jellyfish.... rhopalium (pl. rhopalia)
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