• 1Large living specimens of Lingula unguis (L.) were collected below lowwater mark of neap-tides in the Johore Strait, Singapore.
  • 2The location of the inhalant and exhalant water currents, the subdivision of the mantle chamber into one median exhalant and two lateral inhalant chambers, and the mode of formation of the water currents by ciliary action are described.
  • 3The structure, and also the ciliary feeding and cleansing mechanisms, of the brachia and of the cirri are described.
  • 4The frontal cilia on the adlabial cirri are arranged in five adjacent antagonistic longitudinal tracts. Three tracts beat towards the brachial groove and alternate with two tracts beating in the opposite direction. The frontal cilia on the ablabial cirri are arranged in three tracts, a median tract beating towards the brachial groove flanked by tracts beating away from it. The lateral tracts on both series of cirri beat transversely across the cirri from the frontal surface towards the abfrontal surface; the abfrontal tracts beat distally.
  • 5Lingula unguis is adapted for life on sheltered muddy beaches in its mode of feeding. Small quantities of fine particles are accepted, larger quantities of finely divided matter and coarse particles are rejected. To a considerable extent Lingula selects water-borne particles, which are not bound into mucous threads until they reach the safety of the brachial groove.
  • 6Food particles were collected (a) along the outer margins of the brachial fold, (b) on those frontal tracts of both adlabial and ablabial cirri, that beat towards the brachial groove, and (c) by mass movement of water-borne particles into the brachial groove. All food particles accepted into the brachial groove were carried to the mouth. Due to different conditions in Werent parts of the inhalant stream of water, groups of cirri may operate in unison to effect the acceptance of food particles in one part of a brachium while at the same time other cirri cause the rejection of material.
  • 7Rejection mechanisms include (1) dislodgment of particles already entangled on the frontal surface by strong adverse water currents, (2) themass movement of water-borne particles away from the brachial groove, (3) the shutting of the brachial groove by the brachial fold, and (4) the rejectionof large objects by muscular movements of the cirri.
  • 8The ciliary cleansing mechanisms of the mantle and of the body wall are described.
  • 9Comparisons are made with Crania and with Neothyris.