Undulatory Swimming in a Leech

Wait for the animation to load completely and play through once. To stop the animation, click on the "pause" button. To see the frame that precedes the paused frame, press the "previous" button. To see the frame that follows the paused frame, click on the "next" button. To resume the animation, click on the "play" button.


1) Lay a small clear transparency sheet over the animation. Use two small pieces of paper tape to secure corners of the transparency sheet to the monitor screen. Use a marking pen to make a series of dots on the sheet that track the frame-by-frame position (starting at frame 0) of the leech's head end (right) as it swims through water. [NOTE: Each frame of motion represents 1/30th second of elapsed time (0/30, 1/30, 2/30 etc)]. Make sure the correct elapsed time is recorded for each dot and make sure the distance scale is carefully recorded on the transparency sheet.
2) Remove the transparency sheet and lay it on a piece of white paper. Then, estimate the forward velocity of swimming, as determined by computing the worm's forward progress during a 6/30 sec interval (=1/5 sec). Express units in mm/sec. How fast does the leech swim in terms of body lengths per second? Compare the leech value to those of other annelids worms, such as Lumbriculus..
3) Attach a second transparency sheet over the animation so that the leech image is centered near the top edge of the sheet. Advance the animation to frame 0 and make a tracing of the leech's overall body shape. Label the tracing "f 0". Next, advance the animation to the next frame and shift the position of the transparency sheet about 3 cm upward. Make another tracing of the leech body shape below the first and label it "f 1". Repeat this procedure until you have traced a series of at least six consecutive images, the first positioned at the top and the last at the bottom.
4) The traced images should show that leech swimming is accomplished by production of undulatory, rhythmic waves. Describe the shape of these waves. In what direction do the waves move along the leech's body?
5) Use the tracings you just made to estimate wave frequency.  This is done by determining the time (in sec) it takes to complete one full wave cycle. Then, take the reciprocal of this value to obtain wave frequency, expressed in cycles/sec.

Click here to see non-interactive GIF animation
Software for controlling interactive animations was developed by TOM DREWES