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Otī′tis, inflammation of the internal ear; Ot′ocyst, an auditory vesicle; Ot′olith, a calcareous concretion within the membranous labyrinth of the ear; Otol′ogist, one skilled in otology; Otol′ogy, knowledge of the ear.

But, while the auditory vesicle is usually of a simple round or oval shape in the Invertebrates, it has in the Vertebrates a special and curious structure, the labyrinth.

At first there is a progressive development; the foremost part of the medullary tube enlarges into a brain, and inside this two single sense-organs are developed, a dorsal auditory vesicle and a ventral eye.

However, in the lamprey the spinal cord swells in front into a simple pear-shaped cerebral vesicle, and at each side of it there are a very simple eye and a rudimentary auditory vesicle.