ABSTRACT
Toward the close of the nineteenth century, in a period of rapid development of evolutionary thinking, two hypotheses arose concerning the origin and evolution of the vertebrate ear. These two may be referred to as the "statocyst" hypothesis and the "acustico-lateralis" hypothesis. The first theory of the origin of the vertebrate ear followed naturally from this erroneous conception: the ear was regarded as a development from an "otocyst" presumed to have been passed along the invertebrate series to the prevertebrates. According to a second theory, the labyrinth and ear are considered to have arisen within the vertebrate line itself, being derived from the lateral line organ of fishes. The ostracoderms were primitive vertebrates of the Silurian period, now bracketed with the cyclostomes in the class Agnatha. The ostracoderms already had semicircular canals along with their pore canals, and they lacked the lateral line system.
