Sodium and Potassium in the Endolymph and Perilymph of the Statocyst and in the Eye of Octopus

In the majority of invertebrates the statocyst consists of a single sack; only in the octopod cephalopods is there one sack inside another, so that one can distinguish endolymph and perilymph as in the vertebrate labyrinth. Even in the decapod cephalopods (squids and cuttlefishes) no inner sack is present. The significance of the two sacks has yet to be determined, but it is clearly of interest to discover the composition of the fluids in the two compartments, and in particular whether the endolymph contains high potassium and low sodium concentrations, as in mammals (Smith, Lowry & Wu, 1954; Citron, Exley & Hallpike, 1956; Rauch & Kostlin, 1958). At the same time we have taken for comparison a few samples of fluid from the anterior and posterior chambers of the eye, and from the blood. The endolymphatic and perilymphatic spaces both contain colourless fluids. The perilymph is crossed by numerous fibrous strands carrying blood vessels to the inner sack. These give it a remarkable resemblance under the microscope to the perilymphatic space of vertebrates. In fixed sections the liquid differs in appearance from blood and contains no cells, but often a coarse precipitated material. The endolymph differs in appearance from both blood and perilymph. In sections it also shows a precipitate but no cells (Young, 1959). There is no obvious special apparatus for producing the perilymph. The outer wall of the statocyst is formed of a vascular type of cartilage, not lined by an epithelium. Into the endolymphatic space there opens a canal, representing the remains of the embryonic communication with the ectoderm and known as Kolliker's duct. This is ciliated and might serve for either production or absorption of endolymph. There are muscle fibres in the wall of the sack and in life the contents seem to be under a small pressure. Robertson (1949,1953) measured the sodium and potassium concentrations in the blood and eye fluids of several cephalopods, but so far as we are aware, these analyses have not previously been performed on the body fluids of Octopus vulgaris, nor on the statocyst fluids of any cephalopod.