The great variety and radical metamorphoses of aquatic life forms attracted huge fascination during the nineteenth century, in part because they defied familiar paradigms of development and progress. In this chapter, Cohen explores how writers were inspired by such marine life-cycles to try out experiments in narrative prose, focusing in particular on the influence of marine variety on the depiction of psychological experience. Starting with Charles Kingsley’s Glaucus (1855), Cohen argues that Kingsley uses the life forms of the underwater kingdom to re-energise the poetic figure of metamorphosis, which, in his treatment, depends more upon natural science than myth. Cohen then shows how Kingsley translates marine metamorphosis into narrative experiment in The Water-Babies (1862), and creates an account of psychological experience that is more hallucinatory and phantasmagorical than developmental. Cohen finally suggests that marine metamorphosis has a similar impact on other authors, including Gustave Flaubert, Victor Hugo and Jules Michelet, all of whom stress the disturbing and disruptive possibilities of a psychological prose inspired by aquatic biology.