Hindsight and Rapid Escape in a Freshwater Oligochaete

A novel escape reflex involving the posterior end of a freshwater oligochaete worm, Lumbriculus variegatus, is described. Electrophysiological recordings and videotape analysis from submersed, freely behaving worms show that either a moving shadow or sudden decrease in light intensity evokes repetitive spiking in lateral giant nerve fibers (LGFs) and rapid tail withdrawal when the worm's posterior end is positioned at the airwater interface, to facilitate gas exchange. Because comparable electrical and behavioral response patterns occur in isolated posterior body fragments, but not in midbody or anterior fragments, we conclude that the LGF shadow-sensitivity is localized in posterior segments. Added support for this idea is provided by electron microscopic observations demonstrating the presence of candidate photoreceptor cells in the epidermis of posterior segments. These cells are invaginated distally to form a cavity (phaosome) filled with microvilli, and resemble the known photoreceptors in anterior s...