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The connective tissues surrounding the main central organs and within the musculature and integument of larvae and adults of the lampreys Geotria auslralis, Lampetra fluviatilis and Mordacia mordax have been studied by means of conventional light microscopy, classical and lectin histochemistry and electron microscopy. The results are considered in the context of biochemical investigations to help elucidate the distribution of the different types of collagen in lampreys. The notochord comprises a thin outer layer and very electron-dense ‘elastica externa’, which reacts strongly with Verhoeff's elastic stain, and a thick inner layer of circularly arranged collagen fibrils (diam. 11 nm). The notochord, nerve cord and fat column and the perivisceral cavity are bounded by a continuum of collagenic fibres (fibril diam. 30–40 nm), mainly orientated circularly on the outside and longitudinally on the inside. Comparable fibres extend from this central continuum and the arcualia into the myosepta and around muscle blocks. In the dermis, the collagen fibres (fibril diam. 25–35 nm) are densely packed and arranged in layers, the fibres of consecutive layers running at 90 to 110° to each other. Reticular fibres, which form an extensive network throughout the body, contain fibrils (diam. 20–40 nm) embedded in an amorphous material of medium electron density, characterized by an unusually high content of various glycocomponents. Fibres similar to those of the elastic fibre system of higher vertebrates are present in the subdermis, in the meninges and immediately ventral to the notochord. During the lengthy non-feeding periods of metamorphosis and the upstream spawning migration, the thickness of some collagenic sheaths increases markedly, whereas that of others remains the same or declines precipitously. The significance of these changes is related to differences in the biology of different stages in the life cycle. The connective tissue of lampreys are highly developed and specialized and form an integrated meshwork which, in the absence of vertebral apophyses, must be particularly important in maintaining the structural integrity and functioning of the musculature during swimming.