The colour change in Fundulus is produced by an alteration in the size of its chromatophores, caused by impulses in special nerve fibres running to them. According to Parker (1934 a, b, and elsewhere) the nerves produce their effects not directly, but by chemical transmitters (“neurohumors”) of some kind. The movements of the chromatophores are very rapid, although they are not, as in the cuttlefish and squid, due to muscle fibres pulling the chromatophores out, or allowing them to relax elastically.

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