AbstractCorpuscles of de Quatrefages in the rostral connective tissue of amphioxus have been serially sectioned and examined in the electron microscope. It is shown that each corpuscle is composed of one or a few main cells separated and enveloped by sheath cells. From the main cell two cilia protrude into a lumen formed by a bulge in the sheath. The ciliation of the main cell and its axonic continuation into one of the associated nerves suggests that this cell is a primary sensory neurone and the corpuscles of de Quatrefages probably mechanoreceptors.