phylum echinodermata “spiny” “skin”

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  • Phylum Echinodermataspiny

  • Major Classes of EchinodermsECHINOIDEA: Includes the sea urchins and sand dollarsASTEROIDEA: includes the starfish (sea stars)

  • HOLOTHUROIDEA: includes the sea cucumbersOPHIUROIDEA: includes the brittle stars (below: a fossilized brittle star)

  • Characteristics of EchinodermsCoelomate and radially symmetricalBody is usually 5-rayed (pentaradial)Have a calcareous skeletonPossess a water vascular system used for gas exchange, feeding, and locomotionComplete digestive systemOpen/reduced circulatory system (no heart) with a central ring and vesselsSimple radial nervous system (no brain)Reproductive system consists of gonads which take up a significant amount of space in the body cavitySexual reproduction with fertilization outside body (most)Have ciliated, free-swimming larvaeAbundant on most sea coasts, especially on rocky shores

  • Taking a closer look at Sea StarsPossess 5 or more arms which radiate from a central diskMouth is located on oral surface (underneath)Sea stars are found all over the world in a variety of shapes and colorsMost possess a simple light sensitive eye spot at the end of each ray (arm)

  • DigestionSea stars have 2 stomachs that perform different functions

    The cardiac stomach can be pushed outside of the mouth to engulf and digest food, then digested food is brought inside to the pyloric stomachThe pyloric stomach further digests food and passes it on to the intestine and the anus which is located on the aboral surface (top)

    This allows a sea star to consume food that would normally not fit into its mouth!!!

  • The water vascular system consists of a central ring and a series of canals that eventually end in tube feet which you can see protruding from the underside of this sea star. These tube feet are used for movement, feeding, and bringing in water for gas exchange.The Water Vascular System

  • Here are the tube feet of the sea urchin which protrude out from the spines.Sea stars and urchins alternately contract muscles that force water into the tube feet which extends and pushes them into the ground. The muscles are then relaxed in order for the tube feet to retract.

  • A Closer Look at Sea UrchinsMost sea urchins possess spines of 1-3 cm but can be found up to 20 cm longGloboid shape with no rays or arms

  • Have ball and socket joints on the spines which allow them to move. If you try to touch the spines of an urchin, the surrounding spines will converge there.Possess a mouth with 5 calcium carbonate teeth (see above). Also have a fleshy tongue within. This structure is termed Aristotles Lantern.Sea Urchin mouthSea Urchin Test (shell)

  • Defense systems of EchinodermsSpines are observed on many echinoderms and can be soft or hard and sharp. Their presence deters predatorsPedicellariae are pincer-like structures that protrude from the body surface between the spinesPedicellariae keep the body surface clear of encrusting organismsEchinoderms also camouflage with their natural color into their environment

  • Ecological Roles of EchinodermsStarfish and brittle stars prevent the growth of algal mats on coral reefsOssified skeletons of echinoderms are major contributors to many limestone formationsPart of the food chain, larvae produce food for other creaturesThe death of echinoderms often is followed by an increase in seaweed growth and reef destruction

  • Sea urchin grazing reduces the rate of colonization of bare rockBurrowing of sand dollars and sea cucumbers depletes the sea floor of nutrients and encourages deeper penetration which increases levels of oxygen and ecological tieringSea urchins bore into rocks, releasing nutrientsSea cucumbers provide habitats for parasitesEchinoderms feed on dead and decaying organisms

  • Super Cool Echinoderm FactsSea stars are capable of regeneration of body parts as long as part of the central disk remains

    The gonads of Sea urchins and cucumbers are considered a delicacy in SE Asia

    Sea cucumbers will eviscerate (shoot out parts of their internal organs) when they sense extreme danger, then re-grow the organs

    Sea urchins can replace spines that are damaged or lost


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