Electron microscope observations of the gravity receptor of the tadpole larva of Ciona intestinalis show that the otolith is unicellular. The nucleus of the otolith persists for the entire life of the tadpole. The pigment mass of the otolith is intracellular, and it appears to be built up by fusion of granules. The otolith cell has a free part within the cavity of the cerebral vesicle, and a foot part which is contained within a mound of cells on the ventral wall of the cerebral vesicle. The junction between these two parts is probably the transducer region.
The transducer region is a complex system of folded cell membrane. Distortion of this system during rotation of the tadpole while swimming probably evokes changes in the neurones which surround the foot process of the otolith cell. One of these neurones is connected to the cerebral ganglion by a process which may be an axon. Fibrils extend from the transducer region of the otolith cell to the basement membrane, and probably serve to resist distortion of the transducer region.
The foot process of the otolith cell is connected to the surrounding cells by specializations of the cell membrane similar to attachment plaques.
The observations suggest that the otolith has been evolved from a cilium.