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           SmithLifeScience      Starfish Observation

Echinoderms are invertebrates which are characterized by an external skeleton covered with sharp spines, radial symmetry, and tube feet.  The phylum Echinodermata includes starfishes or sea stars, brittle stars, sea urchins, sea lilies, and sea cucumbers.  All but the last have a limy internal skeleton and hard external spines or plates.  They are fixed or slow-moving inhabitants of the sea, from the high-tide zone to considerable depths.  Often they are abundant but none form colonies.  Species of shallow water are easily collected by hand at low tide and deeper ones are captured by dredging.  
     Starfish may well be the most unusual well-known creature.They have no front or back and they can move in any direction without turning.  Their mouth is on the bottom side called the oral surface.  The top side is called the aboral surface.  Starfish walk using their tube feet to move themselves along a surface. Their tube feet have suckers on the ends, which they use to attach themselves to rocks and to trap prey items. 
     Rather than using muscles to move their hundreds of tube feet, starfish use a complex hydraulic system to move around or cling to rocks. The intake valve for this system, Madreporite, is generally located on the top of the Starfish, just off center.
     Starfish can regrow their arms if they are damaged or eaten by predators. In fact, in some cases an entire sea star can be regenerated from just a single arm!
     Sea stars are carnivores (meat-eaters). They eat clams, oysters, coral, fish, and other animals. They surround the shell and use the suckers on their feet to pull the two shells (or valves) apart. The sea star has enough force in its arms to actually bend the shell! This creates an opening between the two shells that is only .01 inches wide. Using this tiny gap, the sea star puts its stomach out through their mouth and into the clam's shell.  The stomach secretes digestive juices that dissolve the oyster or clam, turning it into liquid. Since starfish have no teeth or jaws, they draw up the liquid through a tube.When it is done, nothing is left but an empty shell.        
     Most species of starfish expel enormous numbers of eggs and sperm into the ocean; fertilization is external. After fertilization, the tiny, transparent, bilaterally-symmetrical larvae (baby sea stars) travel many miles as they are swept along by ocean currents for about two months. As they develop, the tiny larvae swim in the sea, eat phytoplankton, and are a component of zooplankton.
           Common species of starfishes used for class work are Asterias forbesi and A. vulgaris of the Atlantic coast and Pisaster ochraceus of the Pacific coast.

     Kingdom - Animali           Phylum - Echinodermata             Class - Asteroidea


PURPOSE:  To study the external anatomy of a starfish
MATERIALS:  A preserved specimen, dissecting tray, and hand lens.

SeaStarDiagramEL.GIF - 8370 Bytes

     Parts to identify and label:
          Arms or rays - projecting from disc
          Central disc - the center of the animal
          Oral surface  - where the mouth is
          Aboral surface - the top of the starfish
          Madreporite - small white circular area, off-center on aboral surface of disc
          Anus - minute, centered aborally on disc
          Spines - many short, rough, limy, in patterns over aboral surface
          Eyespot - small, pigmented on one end of each arm
          Ambulacral grooves - one along oral surface of each ray
          Oral Spines - surround the mouth
          Tube feet - soft, slender, with expanded tips; 2 or 4 rows in each groove
          Mouth - on oral surface in center



               StarfishDiagram3.GIF - 10611 Bytes

arm2.GIF - 12956 Bytes

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1.     How do sea stars move?

2.     How do they ‘see’?

3.     How does a sea star eat?

4.     What do they eat?

5.     What animals are predators of the sea star?

6.     How do these animals reproduce?

7.     How many arms does it have?

8.     What happens when a sea star loses an arm?

9.     Describe the environment that sea stars like to live in.

10.   What kingdom do starfish belong to?