Lateral undulation
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Lateral undulation is the most primitive of vertebrate locomotor patterns, present even in hagfish, lampreys, and lancelets. It is used both in the water and on land, most notably by snakes in the latter setting.
One can roughly describe lateral undulation as a sequence of alternating left-right waves propagating posteriorly along the animal. This is accomplished by unilateral, posteriorly-propagating muscle activity, which means that, at any point on the animal, only muscles on the left or right side are active, and the active region of muscle contraction moves from head to tail.
In the water, this motion causes the posterior edge of each body-wave to push water backwards, which results in a net forward force on the animal. On land, the same in effect is accomplished, but by pushing on soil, rocks, plants and other irregularities in the substrate.