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Exceptionally little information is available on the nature of pigmentary responses in chelonians. It has been suggested that changes in pigmentation in these animals require relatively long intervals of time and that these changes are morphological in character. Because of the uniqueness of the skin on the carapace and plastron of the soft-shelled turtle, Trionyx, it has been possible to describe the interaction of the various pigment cells in their contribution to the colouration of the integument. In Trionyx, the distribution of the pigment cells is similar to those described for other reptiles. In addition, it has been established that spotting and related patterns are due to increased cell populations and/or differences in the degree of pigment dispersion within the melanin-containing cells. Analysis of the epidermis has demonstrated the existence of functional units of melanin production and distribution-epidermal melanin units. Light adaptation studies have shown that changes in colouration are primarily physiological in nature. With time, these changes may be reinforced by secondary, morphological changes. In vitro and in vivo studies with known melanin dispersing and aggregating substances strongly suggest that the regulatory mechanisms behind these changes are hormonal and not neural in origin.