• 1
    The nymph of Chloeon dipterum is a pelagic animal living in ponds, Its plate-like bilamellate gills are described.
  • 2
    By means of metachronal rhythmical movements of the gills, in which members of pairs beat synchronously, currents of water are set up over the body. These currents which are on the whole axial to the body explose water from all regions surrounding the body.
  • 3
    The gill and abdominal musculature is described and it is shown that a modification of normal musculature has occurred to provide direct muscles to the gill bases and also to make possible the habit of swimming by rhythmical movements of the abdomen rather than by limbs. These movements are lateral and are made about a vertical axis. Swimming is always preceded by a vigorous up and down movement of the caudal filaments. This movemelit about a transverse axis as in the tail flukes of a whale is then abandoned as the side to side movements of the abdomen take charge, when the animal swims aa does a fish.