Glaucus atlanticus, a pelagic nudibranch known by a variety of common names, such as the sea swallow, blue angel, blue dragon or sea lizard. In this illustration, from the "Historia fisica, politica y natural de la isla de Cuba" by Ramon de la Sagra, 1838-1861, it was known as Glaucus radiatus. These nudibranchs float upside-down, with the dorsal view showing the foot.