Animal locomotion can vary in form (e.g., type of appendages used for movement), timing (e.g., time during a tidal cycle when movement is necessary or useful), and scale (e.g., distance an organism is likely to cover per unit time).  Each of these issues influences the effectiveness of a given form of locomotion for strategies of feeding and reproduction.  When considering locomotion, it is also important to consider that the lack of locomotion, or a sessile lifestyle, is vital to the ability of many organisms to occupy and compete for space.

Surprisingly, contractions of the foot muscles that lead to forward motion can involve a wave of contraction either anterior to posterior (retrograde waves) or posterior to anterior (direct waves).  With retrograde waves, contraction of transverse muscles near the anterior edge of the foot force hemolymph anteriorly to extend the foot, and contraction of longitudinal muscles then pulls more posterior portions forward.  This sequence is repeated down the length of the foot in a posterior moving wave.  With direct waves, the posterior portion of the foot is lifted off the surface using dorso-ventral muscles, and then pulled forward by longitudinal muscles, with the sequence repeated in an anterior wave.  A number of molluscs, including polyplacophorans (chitons), prosobranch gastropods (limpets, snails), and opisthobranch gastropods (sea slugs) use one of these forms of creeping locomotion.

a pile of crawling polychaetes

Crawling locomotion--most clearly exhibited by errant segmented worms (polychaetes)-- involves the alternating contraction of longitudinal muscles on the right and left sides of the body to create a sinusoidal body motion that imparts force to the substrate.  In most polychaetes that use sinusoidal crawling, muscles that insert inside the parapodia, and the chaetae that make contact with the substrate, contribute to the force produced for locomotion. Speed of crawling is directly related to the speed of the wave of contraction that passes along the body from posterior to anterior. Polychaetes that crawl along surfaces are highly active and have well developed circulatory systems--watch this movie of blood pumping through the dorsal vessel of a nereid worm.

jointed walking appendages of crabs

Jointed limbs are a hallmark of the arthropods.  In addition to being modified for various rolls in feeding , jointed limbs allow highly precise and controlled movements for walking or swimming of all non-sessile crustaceans at Eagle Cove.  Muscles insert on the internal surfaces of adjacent hard plates of the exoskeleton, and muscle contractions can thereby be used to flex or extend appendages.  For walking, limbs are lifted from the substratum and moved forward (recovery stroke) and then placed down against the substratum to pull and then push the animal forward (power stroke).  Jointed limbs used in walking are generally stenopodous ("slender"), while those used in swimming are phyllopodous ("leaf-like").

the sea star Pisaster ochraceus moves on hundreds of tiny little tube feet

In echinoderms, the water vascular system hydraulically operates the fleshy tube feet (podia) on the oral surfaces of the body. Each podium is fluid-filled and muscular, most often with a sucker at the tip.  Many podia work simultaneously to move these animals along surfaces. The thousands of podia each take alternating power and recovery strokes, making movement of the entire animal smooth.  When moving this way, the body shape of asteroids (sea stars) remains constant given the movement of individual podia along the lower surface of the arms.

Sessile organisms--most clearly exemplified at Eagle Cove by barnacles, sponges, bryozoans, tunicates, and some tube-dwelling polychaete worms--move little or not at all during the adult stage.  Other taxa, such as some bivalves, sea anemones, and other polychaete worms, lead a sedentary lifestyle where movement in the adult stage is possible but typically limited. Most of these organisms are suspension feeders, designed to contact food by pumping and filtering water rather than by movement of the body through space.  Because the site of settlement plays such a vital role in determining physical and feeding conditions throughout life, these animals are typically sensitive to cues that correlate with good settlement sites.