describe how the feet of 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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(for animal cells..)\n1. non-kinetochore microtubules lengthen. this elongates the cell even more than before.\n2. pairs of sister chromosomes attach at opposite ends of the cell.\n3. new nuclear envelopes form around the two sets of dna at the opposite ends of the cell.\n4. cytokinesis (division of cytoplasm) occurs and the plasma membrane re-forms around the two daughter cells.
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I like a very broad question.  Arachnids and crustaceans both shed exoskeletons as they grow.  The arachnids are generally land creatures comprising of spiders, scorpions etc.  Crustaceans comprise of crabs, shrimp, prawns, crayfish, lobster etc.  There are a number of crustaceans that live on land but require water to breed.  The land crab is an example of one.  They also need to shed exoskeletons.\nExoskeletons are what holds there bodies together as they have no internal skeletons.  So as the animal grows, the skin needs to be shed.\nThe arachnid need little water, gathering what they need from the animals that they eat.  The exoskeleton also reduces water lose for those that live in deserts.\nCrustaceans have evolved to live in both fresh and salt water, so the adaptations are to numerous to list.  I hope I have given you a few clues for further investigation.
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between the cell wall and the nucleus.
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About 75 percent scuttle sideways, the better to keep an eye on approaching predators.\n\nAlso, Because that's the way their legs bend. Muscles work in pairs. A muscle can only retract, or pull; to lengthen again it must relax and be pulled back by another 'antagonistic' muscle.
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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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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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Animals have a circulatory system that transports fluids, chemicals, and nutrients around within the animal body. Some plants have an analogous system: the vascular system in vascular plants; trumpet hyphae in bryophytes.\n\nRoot hairs are thin-walled extensions of the epidermal cells in roots. They provide increased surface area and thus more efficient absorption of water and minerals. Water and dissolved mineral nutrients enter the plant via two routes. \n\n\nWater and selected solutes pass through only the cell membrane of the epidermis of the root hair and then through plasmodesmata on every cell until they reach the xylem: intracellular route (apoplastic). Water and solutes enter the cell wall of the root hair and pass between the wall and plasma membrane until the encounter the endodermis, a layer of cells that they must pass through to enter the xylem: extracellular route (symplastic). \n\n\nThe endodermis has a strip of water-proof material (containing suberin) known as the Casparian strip that forces water through the endodermal cell and in such a way regulates the amount of water getting to the xylem. Only when water concentrations inside the endodermal cell fall below that of the cortex parenchyma cells does water flow into the endodermis and on into the xylem.\nXylem and Transport \nXylem is the water transporting tissue in plants that is dead when it reaches functional maturity. Tracheids are long, tapered cells of xylem that have end plates on the cells that contain a great many crossbars. Tracheid walls are festooned with pits. Vessels, an improved form of tracheid, have no (or very few) obstructions (crossbars) on the top or bottom of the cell. The functional diameter of vessels is greater than that of tracheids.\n\nWater is pulled up the xylem by the force of transpiration, water loss from leaves. Mature corn plants can each transpire four gallons of water per week. Transpiration rates in arid-region plants can be even higher. Water molecules are hydrogen bonded to each other. Water lost from the leaves causes diffusion of additional water molecules out of the leaf vein xylem, creating a tug on water molecules along the water columns within the xylem. This "tug" causes water molecules to rise up from the roots to eventually the leaves. The loss of water from the root xylem allows additional water to pass throught the endodermis into the root xylem.\n\nCohesion is the ability of molecules of the same kind to stick together. Water molecules are polar, having slight positive and negative sides, which causes their cohesion. Inside the xylem, water molecules are in a long chain extending from the roots to the leaves. \n\nAdhesion is the tendency of molecules of different kinds to stick together. Water sticks to the cellulose molecules in the walls of the xylem, counteracting the force of gravity and aiding the rise of water within the xylem. \n\nCohesion-Adhesion Theory\nTranspiration exerts a pull on the water column within the xylem. The lost water molecules are replaced by water from the xylem of the leaf veins, causing a tug on water in the xylem. Adhesion of water to the cell walls of the xylem facilitates movement of water upward within the xylem. This combination of cohesive and adhesive forces is referred to as the Cohesion-Adhesion Theory.\n\n\nGuard Cells Regulate Transpiration\nIn most environments, the water concentration outside the leaf is less than that inside the leaf, causing a loss of water through openings in the leaf known as stomata (singular = stoma). Guard cells are crescent-shaped cells of the epidermis that flank the stoma and regulate the size of the opening. Together, the guard cells and stoma comprise the stomatal apparatus. The inner wall of the guard cell is thicker than the rest of the wall. When a guard cell takes up potassium ions, water moves into the cell, causing the cell to become turgid and swell, opening the stoma. When the potassium leaves the guard cell, the water
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They are plant-like protists with two flagella to move. The move in a corkscrew like manner
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Peristalsis.  The sequential contractions of muscles along the canal.
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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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osmosis\nbecause there is a higher water potential inside the seahorse. the sea contains more salt than the seahorse (suprisingly! Lmao) so there is more water in the seahorse. the water molecules travel down the concentration gradient and so the seahorse loses water. though as it is in the sea i doubt this really bothers it
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do you mean what processes cells must undergo? \n\nIf that's your question, then the processes would be cell division by mitosis (to produce more cells, so that organism can increase in size/length), cell elongation(this occurs mainly in plants via vacuolation and thus aids in increase in size and length) and cell differentiation(this allows genes to be switched on or off so that certain traits/phenotypes can be expressed, eg. gene that codes for blue eyes switched on for cell in the eye and not elsewhere like cell on the hand/feet). Hope this answers your question.
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The spiral shell allows the animal inside to grow.  As it grows, the sheel develops a wider turn at the open end and the animal grows into this space.
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Agree with the above. Additional points - the fertilized egg, or zygote, begins to divide, just as in other animal species, still floating freely in the water.  It eventually develops into a ciliated, free-swimming larva called a veliger. The larva swims about in the water, many of them being eaten by small predators like copepods, and filter-feeding organisms. Eventually, those larvae which survive begin to develop the organs of a mature clam, including a mantle, the shell-producing organ.  The mantle then begins to extract minerals, especially calcium, from the ocean water and starts to form the shell.  The weight of the tiny shell carries the young clam to the bottom, where it continues to grow, if it lands on suitable substrate.  However, if the species is a rock-dweller, like marine mussels, and lands on a muddy or sandy bottom, it dies.  Likewise if the larva of a sand-dwelling clam lands on a hard rock bottom, it dies.
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They become excited and expand.  They contract when cold.  See: Standard Temperature and pressure to learn more about what controls gases.
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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