The fine structure of the statocyst of Octopus vulgaris

Abstract

The macula and crista in the statocyst of Octopus vulgaris are composed of hair cells and supporting cells. Below these cells is a nerve plexus, which contains a third type of cell, the “dark” cell. Each hair cell bears kinocilia and microvilli at its distal end. The kinocilia have the typical 9 + 2 arrangement of filaments and in each cell the cilia are arranged in an elongated group of up to 200. Roots of the kinocilia are present in the hair cells and they have banding periodicity of 700 A. The supporting cells bear microvilli at their distal ends and are attached to the hair cells by desmosomes. The plexus contains the “dark” cells, which appear to send processes throughout the plexus. Synapses have not been found upon their perikaryon or processes and it is suggested that the “dark” cells may be a sort of glial cell. Apart from containing the axons of the hair cells, and the processes and cell bodies of the “dark” cells, the plexus contains synapses. These synapses are of several different sorts. There are synapses at the bases of the hair cells or with their processes. There are spines projecting into the hair cells, the spines being post-synaptic, and there are also other synapses involving processes or axons of cells whose identity is uncertain. The synapses are provisionally grouped as collateral or centrifugal.