The anatomy and ultrastructure of the labyrinth of the lamprey ( Lampetra fluviatilis L.)

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Abstract
The anatomy of the labyrinth of the lamprey (Lampetra fluviatilis) is described. The ampullae of the two semicircular canals are each equipped with a complex three-armed sensory crista. They may be considered homologues of the ampullae of the vertical canals of the gnathostomes. However, the complexity of their cristae encourages the assumption that they may cover a spatial range of responses to angular acceleration which includes responses to accelerations in a horizontal plane controlled in the gnathostomes by the horizontal semi-circular canal. The otolith-bearing end organs are found to be located on a common macular structure. This is subdivided into an anterior horizontal, a vertical, and a posterior horizontal macula, each of which portions carries a characteristic arrangement of sensory cells. On the basis of an electromicroscopic analysis of the orientation of the hair cells in the three main portions of the macula a revision of the homologies found in the older literature appears to be called for. It is suggested to homologize the anterior horizontal macula with the macula utriculi, the vertical macula with the macula sacculi and the posterior horizontal macula with the macula lagenae of the labyrinth of gnathostome animals. A separate sensory ending in the dorsal part of the labyrinth, the dorsal macula, may be the homologue of the macula neglecta. Ultrastructurally the end organs of the lamprey labyrinth conform with those of the gnathostome labyrinth with the exception of the presence of a new type of sensory hair cell which is equipped with a stiff kinocilium of extraordinary length accompanied by extremely short stereocilia. This cell is found preponderantly in the vertical macula (macula sacculi). A striated organelle in the cytoplasm of the hair cells appears to be uniquely confined to the labyrinth of the lamprey. Morphologically the lamprey labyrinth differs from all other chordate labyrinths including that of its fellow cyclostome Myxine by the presence of large ciliated chambers in its centre in which long and powerful cilia maintain a permanent pattern of four endolymph vortices. The ciliated chambers are in open communication with the ampullae and with the spaces containing the otolith-bearing maculae. The analysis of the functional significance of the anatomical and ultrastructural findings will be described in a separate paper.