The epidermis of the eel A. Anguilla L. secretes a mucous layer mainly from its mucous cells although there may also be a contribution from surface epidermal cells . Under certain conditions, the epidermal cells may transform to a secretory cell type that resembles, but is structurally distinct from a mucous cell . This observation has not previously been reported in fish epidermis. Specialised club cells, characteristic of the Anguillidae contain filamentous material that may be secreted beneath the epidermal surface. This previously unobserved phenomenon together with an increase in cell numbers around the lateral region may suggest an additional protective mechanism which may be of use, for example, during 'burrowing'. The quantitative distribution of mucous cells over the body was also determined and the cellular contents of the main cell types characterised histochemically. Lymphocytes and previously unseen granular neutrophil-like cells may function in more specific protective mechanisms against pathogenic organisms . The mucus has been examined biochemically and separated by chromatography and electrophoresis to reveal several components. Rheological study has established shear thinning properties of the mucus and functional groups essential to maintain the integrity of the mucous structure. The dynamics of mucous secretion have been observed autoradiographically and a model for the mechanism of secretion proposed . Attempts have been made to measure the rate of mucous secretion under a number of experimental conditions which are discussed in relation to the viscous nature of mucus and the proposed model for secretion. The epidermis and its mucous secretion thus provide an effective lubricative barrier to the environment extremely important for locomotion and protection from dessication, osmotic stress and disease organisms.

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