1. The six pairs of gills in Ephemera danica causing currents in the water are bilamellate and feather-like.

2. The gills move in metachronal rhythm from before backwards, and set up currents which are symmetrical with the body axis. Members of pairs of gills beat synchronously and in phase with each other. The difference of phase between two adjacent gills of the same side is about one-eighth of a complete oscillation.

3. For a single gill there is little or no difference in the angle presented by the gill to its own path of motion, in the two halves of an oscillation. This factor (angle of incidence) is not of great importance in directing water backwards. Forces of importance in moving water in the direction taken are (i) the resultant of axial and transverse force components for single gills, (ii) the pressure effected on the water over the animal's back by the two members of a pair of gills falling, (iii) the movement backwards of alternating pressure regions over the back, (iv) the undulations of the second screw-like gills which feed the main current from in front and from the sides.

4. The fringing filaments of adjacent gills overlap each other from before backwards (as seen from the middle line of the body), and thus each row of gills forms a "membrane" composed of separate gill units. The latter appear never to lose contact with each other during movement, and since no sideways flow is observed between the gills it is assumed that the overlapping filaments between the gills form a membrane impermeable to flow.

5. The adaptation of Ephemera to its sandy environment is shortly discussed.

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