STARFISH DISSECTION PREP

MARCH 24TH, 2011

Starfish Main IdeasStarfish hunt bi-valves (clams, oysters, and mussels). These organisms are very strong and have shells, so a starfish needs to be able to tire them out. They therefore use water pressure and not muscles to move, because muscles get tired first.

Once they have tired out their prey and opened the shell some, they invert their first stomach into the prey and eat it inside out. Dissolved food goes into the second stomach for more digestion, and then into the intestine to be absorbed.

Because this is happening underwater and its messy, they need lots of stomach acids and other digestive chemicals. Most of the space in their bodies goes into making these.

Starfish have radial symmetry and can move, attack, or defend in any direction… slowly. They have an ‘eye’ at the tip of each leg to sense light. They have no brain center but lots of nerves that give them a good sense of touch.

Dissection Write-Up1) External anatomy2) Internal anatomy3) Diagram of external ventral side4) Diagram of a cross section of a leg5) Diagram of the top layer of internal anatomy (mainly digestive organs)6) Diagram of the bottom layer of internal anatomy (reproductive organs and

leg structure)7) Data table:

a) Group nameb) Radius of starfishc) Mass of starfishd) Mass of digestive organs

9) Scatterplot of radius(x) vs. starfish mass (y)10) Scatterplot of starfish mass (x) vs. digestive mass (y)

All diagrams should include labels.

Starfish Diagram

EXTERNAL ANATOMYArms – locomotion, capturing preyCentral Disc – site of main organsSieve Plate (Madreporite) – filtered water intakeTube Feet – Used for gripping, moving, huntingSkin Gills (Dermal Branchiae) – diffusion of oxygen and carbon dioxidePedicellariae – miniature pincer stalksAmbulacral Groove – centered in arm, it has up to 4 rows of hydraulic spines.Ambulacral Spines – powered by water pressure, use: locomotion, capturing prey.Mouth – ventral opening of digestive trackAnus – dorsal opening of the digestive trackWater Vascular System – used for locomotion

http://www.pgjr.alpine.k12.ut.us/science/whitaker/Animal_Kingdom/SeaStar/SeaStar.html

http://www.k-state.edu/organismic/echinoderms_and_protochordates.htm

Starfish Diagram 2

INTERNAL ANATOMYIntestine – absorb digested foodDigestive Glands – create chemicals to digest foodGonads – reproductive organsCardiac Stomach – can be inverted out through the mouth to eat foodPyloric Stomach – second stomach connected to the cardiac stomachStone Canal – connects the sieve plate (madreporite) to the ring canalAmpulla – water nodules which connect to suction feetOssicle – provides support and structure to armsRadial Canal – water channels going lengthwise along each armRing Canal – ring connecting all five water channels to the cardiac stomach

Suggested Incisions1) Find mass with scale2) Observe external features3) Pin ventral side down4) Cut off the tips of 2 legs5) Cut dorsal skin up 2 legs6) Carefully connect the incision between the 2 legs, and proceed around

the middle. Remove dorsal skin over middle.7) Observe top layer of internal features8) Remove digestive organs but not reproductive organs.9) Observe bottom layer of interal features10) Find digestive organ mass with scale