Statocyst
Balancing organ in certain aquatic invertebrate animals
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Through Invertebrate, Inertia and Gravity
In Mnemiopsis leidyi, volumetric electron microscopy of statocysts found a synaptic nerve net (the aboral nerve net, or ANN) that synapses onto the mechanosensory balancer cells while receiving no input from them. The ANN coordinates the timing of ciliary arrest and re-beat across its four balancer quadrants, with "bridge cells" that span the statocyst providing the only feedback to the network.
As a result, the statolith shifts as the animal moves. Any movement large enough to throw the organism off balance causes the statolith to brush against tiny bristles which in turn send a message to the brain to correct its balance.
It may have been present in the common ancestor of cnidarians and bilaterians.
Hearing
In cephalopods like squids, statocysts provide a cochlea-like mechanism to hear. As a result, the longfin inshore squid for instance can hear low-frequency sounds between 30 and 500 Hz when the water temperature is above 8 °C (46 °F).