INTRODUCTION: Ifnumbersaloneareusedasameasureofsuccess,thejointed­ leggedanimalsarethemostsuccessfulanimals. Theirbodysegmentationsuggeststhat theyevolvedfromsegmentedworms. Thesegments,however,aremorespecialized,and manysegmentshaveanappendage.

Senseorgans,suchaseyesandantennae,musclesarrangedingroups,jointed

legs,andaprotectivebodycoveringhavemadeitpossiblefortheseanimalstobecome moresuccessfulthananyothergroupofanimals...Althoughjointedlegged animalsvary greatlyinstructure,thebasicbodyplanisthesame.

Thecrayfishisalargeaquaticarthropod,whichmeansitcarriesitsskeletonon theoutside. Becauseofitssize,andrepresentativeanatomyofthejointed-leggedanimals,itisausefuldissectionspecimen. Pleaseusethisexperiencetostudythe generalorganization oftheexternalanatomyofthisgroupofanimals.

PURPOSE:Toexaminethecrayfishasarepresentativeexampleofthejointed-legged animalsandtobecomefamiliarwithitsexternalandinternalstructures.

EXTERNALANATOMY PROCEDUREANDOBSERVATIONS:

APutongoggles andgloves. Placethecrayfishinthedissectingtraywith

itsdorsalsideup. Likeallcrustaceans,acrayfishhasanexoskeletonthatcoversits body. Lookattheexoskeletonandfindthetwosections,thecephalothoraxandthe abdomen. Thepartoftheexoskeletonthatcoversthecephalothoraxiscalledthe carapace. Theabdomenisbehindthecephalothorax andisalsodividedintosections. Countthem. Theabdomenendsinasegmentcalledthetelson.

1. Howmanysegmentsdoesthecrayfishhaveinitsabdomen?

B.Thecarapaceextendstowardthefrontoftheanimal'sbodyandformsahardbeak calledtherostrum. Findthecompoundeyesthatliebeneaththerostrum. Examine theeyeswithahandlensandseeifyoucanobservethemanylensfacetsthatmake upacompoundeye. Observethediagramsandmodelsoflensfacets/compound eyes providedbyyourteacher.

2. Whatisthedifferencebetweenacompoundeyeandyoureye?

C.Theappendagesclosesttothefrontoftheanimal'sbodyarebranchedantennules.

Locatethelargeantennaejustbehindantennules. Theantennulesareorgansusedfor balancetouchandtaste. Longantennaeareorgansusedfortouch,taste,andsmell.

3. Howmanyantennaedoesacrayfishhave?

4. Whatisthedifferenceinstructureoftheantennaeandantennules?

5. Whatisthedifferenceinfunctionoftheantennaeandantennules?

D.Turnthecrayfishoverandexaminethestructuresonthebottomoftheanimal's body.

Locatethemouth.andthe.mandibles(jaws)...Trytomovethemandibles and seehow

theymoveinrelationshiptothecrayfish'smouth/face. Themandibles,orjaws,crush foodbymovingfromsidetoside.

6. Describethe structureofthemandibles.

7. Inwhichdirectiondothemandiblesmove?

8. Howisthisdifferentfromthewayyourjawsmove?

E. Looknearthemouthandfindthethreepairsofmouthpartsknownasmaxillipeds.

Theseareused duringeating. Twopairsofmaxillipedsholdsolidfood,tearit, and passittothemouth. Thethirdpairofmaxillipedsdrawswateroverthegills.

9. Howdoestheshapeofthemaxillipedsdifferfromthewalkinglegs?

F.Thenextpairofappendagesisthemostobvious. Theseareclaws. Thechelipedsare usedfordefenseandtocaptureprey.

10.Howisthechelipedshapedtofititsfunction?

G.Thenextfourpairsoflegsarecalledwalkinglegs. Examinethemcarefully.

ll.Howdothewalkinglegsdifferinshape?

12.Whatarethefunctionsoftheappendagesthatarefoundonthe cephalothorax?(alloftheonesyouhavealreadyobserved)

H. Now,observetheappendagesontheabdomen. Wecanusetheseappendagesto determinethesexofyourcrayfish. Thefirstfivesegmentsoftheabdomeneach haveapairofswimmerets,whichcreatewatercurrentsandfunctioninreproduction. Alloftheswimmeretsinthefemalearesmallandsoft. Inthemale,thefirsttwopairs arehardandmodifiedfortransferringsperm. Now,findtheplacewherethewalking legsattachtothebody. Ifyouthinkyouhaveafemale,youcanuseamagnifying glasstofindthecrescentshapedslitthatisthegenitalporenearwherethe3'dpairof walkinglegsattaches. Inamale,thespermductopeningsarefoundwherefourthpairofwalkinglegsattaches. Afterdeterminingthesexofyourspecimen,besureyouhaveobservedtheoppositesex.

veins. Thebloodflowsoverthegillsbeforereturningtotheheart. Removetheheart andcutoutthesidesofthethoraxtoexposetheorgansunderneath. Thetwolight­ coloredmassesextendingoneachsideofthebodyintotheheadaredigestiveglands. Theyproduceanacidthatisneededintheintestinestochemicallybreakdownfood thecrayfisheats.

19.Howdothedigestiveglandshelpindigestion?

20.Whyisitgoodtohavethedigestiveglandsattachedtotheintestine?

C. Betweenthesedigestiveglandsyoushouldseeasmall pairofwhitereproductive organsinthemaleanimal. Thefemalewillprobablyhavealargemassofdark­ coloredeggs. Theseeggsmustberemovedbeforeproceeding. Tolocatethe intestine,insertthepointofthescissorsunderthetopsideoftheshellandcutbackto thetelson. Spreadtheshell,andtheintestinewillbefoundasatubeonthetopsideof themusclesoftheabdomen. Traceitforwardtothepointwheretheintestinejoins thelarge,thin-walledstomach. Cutintothestomachandexaminetheinteriorwalls. Locatethehard,tooth-likestructureswhichfunctioninmechanicaldigestionofthe foodthecrayfisheats. Whatdoesthismean? Wheredoesthishappeninyourbody? Yourcrayfishmayalsohavefoodremaininginitsstomach!

21.Howdothehard,tooth-likestructuresaidthestomachindigestion?

D. Now,removealloftheorgansbycuttingtheshortesophagus belowthestomachand thebandsofmuscleholdingthestomachjustbehindtheeyes. Youshouldbeableto liftoutmostoftheinternalorgansinonepiece. Cleanouttheremainingtissueinthe headsothatthegreenglandsareexposedjustbehindtheantennules. These

structuresfunctionlikeyourkidneys. Whatdoesthismeanthattheydoforthe crayfish?

22.Whatcolorarethegreenglands?(TheyareNOTgreen.)

23.Whatisthefunctionofthegreenglands?

E. Inthefrontpartoftheheadcavity,betweentheeyesisasmallmassofwhitetissue, thebrain. Tracethenervesthatgofromtheantennaeandtheeyes. Thentracethe nervecordbackfromthebraintotheabdomen. Theenlargementsofthenervecord ineachsegmentoftheabdomenaretheganglia.Youshouldbeabletofindthenerve cordthatrunsontheventral(bottom)sideofthecrayfish. Thecrayfishusesits abdomen(tail)fordefenseandcancurluporusethetailtoquicklyswimawayif threatened.

24.Whyisthecrayfish'snervecordonthebottomofitsbody?

Pleaseanswerthefollowingquestionsincompletesentences.

EXTERNALANATOMY:

1.Howmanysegmentsdoesthecrayfishhaveinitsabdomen? (Do not count the telson and uropods)

2.Whatisthestructuraldifferencebetweenacompoundeyeandyoureye?

3.Whatisthedifferenceinstructureoftheantennaeandantennules?

4.Howmanyantennaedoesthecrayfishhave?

5.Whatisthedifferencebetweenthefunctionoftheantennaeandantennules?

6.Describethestructureofthemandibles.

7.Inwhichdirectiondothemandiblesmove?

8.Howisthisdifferentfromthewayyourjawsmove?

9.Howdoestheshapeofthemaxillipedsdifferfromthewalkinglegs?

10. Howisthechelipedshapedtofititsfunction?

11. Howdothewalkinglegsdifferinshapeasyoumovefromfronttoback?

12. Whatarethefunctionsoftheappendagesthatarefoundonthecephalothorax?(allof theonesyouhavealreadyobserved;useasentenceor2todescribethese)

13.Whatisthesexofyourcrayfish? (male or female…not boy or girl)

14. Howdidyoudeterminethesexofyourcrayfish(whatstructuresdiditpossess)?

CrayfishExternalAnatomy

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(Sectionofthebody)

(Sectionof thebody)

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INTERNALANATOMY:

16. Whyarethegillsbranched(thinkabouttheirfunction)?(2PTS)

17. Towhichstructuresarethegillsattached?

18. Whatisthe advantageofthegillsbeingattachedtothewalkinglegs?(2PTS)

19. Howdo thedigestiveglandshelpwithdigestion?

20. Whyisitgoodtohavethedigestiveglandsattachedtothe intestine?(2PTS)

21. Howdothehard,tooth-likestructuresaidthestomachindigestion?(2PTS)

22. Whatcolorarethegreenglands?(TheyareNOTgreen.)

23. Whatisthe functionofthegreenglands?

24. Whyisthenervecordonthebottomofitsbody?(2PTS)

INTERNALANATOMYOFACRAYFISH