describe how the foot of a sea urchin expands and contracts?
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Sea urchin belongs to a group of animals called: echinodermata (among them: starfish, brittle star and sea cucumbers). All echinoderms have a water-vascular system, a set of water-filled canals branching from a ring canal that encircles the gut. The canals lead to podia, or tube feet, which are sucker-like appendages that the echinoderm can use to move, grip the substrate, or manipulate objects. These tube feet are extended and retracted by hydraulic pressure in the water-vascular system. Pedicellaria are small, snapper-like skeletal elements that are used by the echinoderm to keep small organisms from settling on its body.
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Starfish prolapse their "stomach" inserting it into the shells of their favorite food, clams/mussels.  This way the starfish can digest/ingest the meat while the prey is still inside the shell.  \nSome sea cucumbers expel their stomach and other inner organs (which are easily regenerated) when being attacked.  The hope is the predator will be distrated by the insides and the sea cucumber can get away.
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They're critters that permanently attach themselves to something and live out their lives there, like many sea creatures- usually they are motile as larvae and then settle on an object as an adult. Very few are sessile their entire lifespan but many many spend their adulthood that way.\n\nExamples- barnacles, sponges, corals\n\nWhen applied to plants (I assumed you meant animals, but just in case... ) it refers to leaves or stems that grow directly from the stem without a stalk.
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Briefly you apply force to spin something - petal\nwhen it spins the insides experience       - fugal\n\nbut the links will explain it in more detail
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Diaphragm:  Expands downwards into the abdominal cavity to allow for the lungs to expand as we inhale. As the diaphragm contracts the air is expelled from the lungs.\n\nRibs:  The ribs protect the lungs and expand as you inhale to fascillitate space for the lungs to expand like the diaphragm.  The ribs then contract expelling the air in the lungs.\n \nAltogether now:  The diaphragm relaxes and expands downward into the abdominal cavity as the ribs relax to also expand for an increase in air space for the lungs to take in O2. The ribs and diaphragm then contract to expel the CO2 and waste products from the de-oxygenated gases as you exhale.
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Surface tension, or the attraction of water molecules sideways and inwards at the surface of the water, cause water to act like an elastic trampoline. This lets the animal float without breaking the surface.\n\nTo propel itself forward, the insect's feet must push water backward (for every action of the bug there is an equal and opposite reaction on the water). The rowing action of the legs produces miniature vortices (whirlpools) which move backward from the bug's feet.
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A thick filament of myosin lies near the thin filaments of actin.  In the sliding mechanism, the thin filaments slide into the thick filaments, producing the contraction
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They adapt to the water around them.
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All animals move at some point in their life.  However, animals like sponges, hydras, anemones and coral are stationary in their adult form.
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Like a stretched spring.  Refer link for image.
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Invertebrates, the starfish can turn its stomach inside out. It will the surround its prey and digest the food.  When the food is digested the stomach will return to its normal position.\n\nNasty eh?
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Read the information on these sites and maybe you will find some of the answers to your questions.\n\nhttp://www.answers.com/echinoderm%20\nhttp://www.answers.com/topic/tube-foot?hl=echinoderm\nhttp://www.answers.com/Aristotle%E2%80%99s%20Lantern\nhttp://www.answers.com/main/ntquery?method=4&dsid=2222&dekey=Sea+urchin&gwp=8&curtab=2222_1&linktext=sea%20urchins
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There is a chart for the hand and a chart for the foot. See links below.
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Sand dollars are of the Phylum Echinodermata, class Echinoidea.   They, like the sea urchin, have no arms or legs  but move around by tiny spines on their body.  Sand dollars are usually found lying in a bed buried under a layer of sand.  If a sand dollar is found alive it will appear to have a layer of very fine hair on its body.  These are the spines.   They are a slow moving grazer that feeds on disintegrating organic material found within their sand beds.
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Very minute claws on the feet that allow them to grip. Some insects actually have a sticky substance on their feet that allows them to stick to surfaces.
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Earthworms travel underground by the means of waves of muscular contractions which alternately shorten and lengthen the body. The shortened part is anchored to the surrounding soil by tiny claw-like bristles (setae) set along its segmented length. (Typically, earthworms have four pairs of setae for each segment but some genera are perichaetine, having a large number of setae on each segment.) The whole process is aided by the secretion of a slimy lubricating mucus
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the contraction and release of the muscles