Snails can’t move backwards. The muscles in their foot can only wave in one direction. They could make long and gradual u-turns, I suppose, but they can’t reverse over the slime trails behind them, which they use for communicating with each other and finding their way back home. In that sense, these slow-motion gastropods are the living, oozing embodiment of what Søren Kierkegaard once wrote about the world as we experience it: “Life can only be understood backwards, but we have to live it forwards.” What a shame the father of existentialism is a hair too dead to hear those words cited at a crucial moment in Adam Elliot’s wise and wistful “Memoir of a Snail,” as Kierkegaard — so fascinated with the anxiety and despair of the human condition — would’ve admired the obsessive toil of stop-motion animation, particularly in the service of such a poignant tribute...