Plankton move through
the action of cilia (see the ciliary bands, which appear on the edges
of body lobes in the sea cucumber larva to the left), antennae
(as seen in the copepod),
jointed appendages (as in the crab
larva), muscle contractions in the body wall (as in annelids)
and/or jet propulsion (as seen in this jellyfish
movie). Because the plankton are heavier than seawater, they depend
on these forms of locomotion just to stay in the water column (the density
of water is a constraint on the distance these small organisms are able
to move). Movement of flagella works by the flagellum beating back
and forth or in a helical rotary pattern. Cilia often occur in distinct
tracts and move together in waves, as seen in this ctenophore
movie. The ciliary movement provides a uniform pulsing force that has forward
(power) and recovery strokes.