1. The otolith of the hake (Merlucius merluccius Linn.) is a compound structure, comprising an organic and an inorganic constituent.

2. The organic constituent is the more complex in structure, and consists of concentric shells (appearing in sections of the otolith as lamellae) which have a reticulate structure, and are probably fibrous in nature. The concentric shells are separated at a fairly constant distance of about 2µ from one another, and are bound together by comparatively stout radial fibres.

3. The inorganic constituent consists of needle-like crystals, about 40µ in maximum length, and less than 1µ thick, which are secreted among the radial fibres, and pass normally through the concentric shells of the organic constituent from the centre of the otolith to the periphery. These crystals interlock to give the otolith its very solid structure.

4. The concentric rings, apparent when the entire otolith is viewed lying on its side, and which, in some species of fish, are used for the assessment of age, are due to the varying thickness of the concentric shells, described in (2) above. These are, typically, grouped in such a way that there are alternating zones where thick and thin concentric shells (lamellae) predominate. Where thick lamellae predominate an effect of opacity is produced, where thin lamellae predominate, one of comparative translucency. The thick lamellae are of the order of 1.5µ at their thickest, the thin lamellae extremely tenuous.

5. The cause of the ringed structure in the entire otolith is therefore due to the organic constituent, the inorganic constituent (crystals) having a supporting function only, in the otolith.

6. Immermann's hypothesis, that the rings are due to the structure of the inorganic constituent, being an optical effect produced by an internal reflection of the light, caused by a twisting of the crystals, is not accepted as applying to the otolith of the hake.

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