Dissection Guide for the Clam (Mussel) 07.0s.15Name Partner Date Hour

Mollusksare soft-bodied invertebrates. They have a muscular foot anda mantle.In mostmollusks, the mantlesecretes a hard shell.In this investigationyou will observethe externaland internalstructures of a representativemollusk--the clam or fresh-watermussel. Clams are pelecypods, or bivalves,and havea two-parthinged shell. Clams are found in freshwater in streams,ponds, and lakes. They also are very common burrowedinto the mud of oceanmud flats. Clamsare often used for food.

Clamsbelong to the phylumMollusca. Molluscs(Latin molluscus,"soft"), asthe origin of the namesuggests, aresoft-bodied animals having an internalor externalshell. Included in the phylumare snails, oysters, slugs, clams,octopuses, and squids. Most molluscsare bilaterally symmetrical (have a left andright side)and have well-developedrespiratory, excretory, circulatory, and digestive systems. Some may havea calcareousshell surroundingthe bodymass.

Molluscsare similar to annelidsin their development.Both havetrochophore larvae. Molluscs differ from annelids,however, in the absenceof segmentation.Further, the coelom,so prominentin the annelids,is greatly reducedin the molluscsand is generallyrestricted to an areasurrounding the heart.

Most molluscsare slow moving,but the bodiesof severalspecies have been highly modifiedfor rapid locomotion.Although primarily marineorganisms, some molluscs are found in freshwater (clams and snails) andon land (snailsand slugs).

The molluscsare characterized by havingthree main body regions: a head-foot,which is the sensoryand locomotivepart of the body; a visceralmaFs containing the excretory,digestive, and circulatory ofgans; and the mantle,which secretesthe shell.The gills, which functionin respiration,are located inside the mantle.

To what phylumdoes the clambelong? Whatdoes the nameof the phylum translate(from the Latin language)to mean? List six commonmembers of the phylumMollusca:

What doesit meanwhen an organismis bilaterallysymmetrical? What element(from the PeriodicTable) is a majorcomponent of a calcareousshell? Namethe threemain body regions of a mollusk:

Clamsbelong to the classPelecypoda (which means "hatchet foot"). Membersof the classpelecypoda provide delightfor epicureans,jewelers, and artisans the world over,because they providefood, pearls, and mother-of-pearl,which canbe fashionedinto hundredsof forms.Another name for this groupis "bivalves,"as they possesstwo shellsor valves.Included in the Soup areclams, oysters, mussels, scallops, and shipworms. They vary in sizefrom onecentimeter across up to well overone meter (the Giant Clam of the SouthPacific). Sinceclams and mussels are found both in salt andfresh water, they arecommon throughout the United States andthe entireworld. To what class doesthe clam belons? What doesthe name of the classtranslate (from the Latin language)to mean? -- What does it mean if an animal is called a bivalve? An epicureanwould be interestedin List five common membersof the classPelecypoda:

External Anatomyof the Clam

Obtaina preservedclam and rinse it thoroughlyto removeexcess preservative. Place the clamin a dissecting tray.

Observethe bivalveshell. Notice the hingeligament The small,pointed area near the hingeligament is calledthe Z)2,.t /4//ll umbo.It is the oldestpan of the chin.The umbois situated dorsallytoward the anteriorend of the clamand is surrounded =,/l by concentricgrowth lines. The linesrepresent alternating periodsof slow andrapid growth. >z

Beforeyou continuewith this investigation,it is importantto know the orientationof the clamshell. Recall that the umbois nearthe anterior end. The posterior of the clam shellis at the oppositeend. In referenceto the clamshell, dorsal is the side,or edge,with theumbo. Ventral is the side,or edge,opposite the umbo.Locate the posterior,anterior, dorsal, and ventral surfaces of your clam shell.Hold the clamshell with the anteriorend up andthe hingefacing toward you. Locatethe posterior, right valve,and left valveof theclam shell.

Describethe locationof the hingein relationto the two shell-halves. What is the nameof the oldestpart of the clam shell? ATTEMPT (sometimesthis is difficult) to countthe numberof concentricgrowth lines (think abouttree rings) on the shell. What is vour bestestimate? On the following drawingsof the clam shell,label the following: dorsal,ventral, anterior, posterior, right valve, left valve,and hinge. Theterms can be usedmore than once if necessary. Tagthe following: (Note: Do not try to pin the shell. Makeyour tagsand "stick" themto the partswith moisturefrom the shell.)

Dorsal Ventral Anterior Posterior Right valve Left valve Umbo

Verified Hold the clam in the dissectingtray as shown in the figure at the right. With a scalpel carefully scrape,away some of the horny outer layer of the shell. Scrapeuntil you seethe "white part" (prismatic layer) of the shell-it doesnot need to be a very large area. CAUTION: Scrape in a direction away from your hand to avoid cutting yourself.

Completed

The shellof a clam is madeup of threelayers: the homy outer layer,the thick, middlelayer called the prismaticlayer, and the innermostlayer called the pearlylayer. Haveyour instructorplace one drop of acidon the exposedprismatic layer.CAUTION: Do not let any acid contactyour skin to Flg. '$'2 avoid acid burns. The bubblingof the acid indicatesthat calciumcarbonate (CaCOl)is present. Carefully rinse the shellwith wateronce you havemade your observations.

Describe what happenedwhen acid was placed on the exposedprismatic layer: Describea situationwhere what you observedmight actuallybe appliedto the clamin its naturalenvironment:

Name the material (chemical) that makesup the clam shell: Give the chemical formula of the material: Here is some extra information on the make-up of the clam shtill: How many layers make up the "shell" of the clam? How manylayers make up the "mantle"of the clam?

Periostracum Which part of the clam shellforms pearls? Prismaticlayer Shell A pearlis formedas a reactionto a

Nacr€ouslayer betweenthe shell and mantle. - Nacre-secretingcells I The white part of the clam shell (you ( .- Conneclivetissue uantre applied the acid to in the exercise 'ir.,.,i ir.*'-' I ,,i....,lti i.", I ^\/.- ;'i; . I above) is the Ciliatedepithelium ) Epitheliumis betterknown as by its

Foreignbody (sand or a Parasite) Pearllormed bY secretion commonname: belweenshell and mantle o{ nacre around loreign bodY **Put on a pair of safetyglasses during the processof opening the muSsel.

Internal Anatomyof the Clam:

Your clam shouldbe slightly gappedopen. Theremight evenbe a pieceof woodwedged between the valves- this guaranteesthe preservativegets inside the shellhalves. Gills Try first to pry it openwith your hands.If this will not work, usea screwdriverto very gentlypry the valvesapart. Your clamshould look like the oneshown to the right.

Look at the partiallyopened shell. Observe the anterioradductor muscle,posterior adductor muscle, mantle, and foot. The opening betweenthe two shellsis calledthe gape.Carefully insert the scalpelbetween the mantleand the left valveof the shell.Cut the anterioradductor muscle as close to the shellas possible. CAUTION: The scalpelis a sharpinstrument. Always be very carefulwhen handlingit and cut awayfrom your hand and body. Repeatthis procedureto cut the posterioradduitor muscle.Open the shell.If necessary,carefully run your fingersor scalpelbetween the shelland the

LINEOF OROWIH mantleto separatethe mantlefrom the shell.The spbcebetween the two halves of the mantle is the R ADDUCIOI gts ferio r il;;:fa' mantlecaviw. ADDUCTORMUSCLE sCAI.^PETSLADE Openthe left valve asfar aspossible. When done, your specimenshould look like the diagrambelow.

Mantleline Anltrior' adductorscar

Completed(the two shellhalves are open as shown to the right).

Verified

Observethe hinge.Notice the interlockingteeth that hold the two valvesof the shelltogether. Locate the "scars"from the anteriorand posterior adductor muscles on the innersurface of the left valve.These scars indicate where the posterior and anterioradductor muscles were attached.

What is the gape? What formsthe "scars"on the valves?

Describe the inner layer of the shell. hjeril. r adductor muscle .l Lets stop for just a moment on this page and look at some of the living clam's anatomy. The diagramsbelow show how he looks when healthy and alive. The foot and siphonsare extended. Yours will not-look exactly like this becauseit is dead and theseappendages have been retractedinto the shell.

UMSO

VALVE(SHELL)

Hereare some terms you will needto becomefamiliar with to continuewith the dissection.Spenda few minute studyingthem-it will help!

Stnrcture Description/l,ocation Function Anterioradductor muscle Anterioredge inside shell Holdstwo halvesof shelltogether Posterioradductor muscle Posterioredse inside shell Holdstwo halvesof shelltosether Mantle Membranoustissue that covers Secretesshell entirebody; yellow or cream- eolored Foot Hatchet-shaped;hard; anterior and Locomotion.movement ventralto sills Gills Folded,ridged tissue with micro- Respiration,gas orchange scopiccilia hcurrent srpnon t old ln mantle;posterior end ven- Regulatesflow of water into clam tral to excunentsiohon Fxcurrentsiphon Foldin mantle;posterior end; dor- Regulatesllow of water out of clam salto incurrentsiphon Palps leatlike structuresanterior to gills Directswater carrying food into mouth and posteriorto anterioradductor muscle Mouth Slit betweenpalps Passageof food into dicestivesvstem Stomach Saclikestructure near mouth Disestionof food Intestine Coiledtubule from stomach Absorptionof digestedlood throushbodv to anus Digestivegland (liver) Light greenmass surrounding Secretesenzymes into digestivesystem stomach to disestfood Anus End of intestinenear excunent Removalof undigestedfood siohon Reproductiveorgan Spong reddishmass ventral to Productionof eggsand sperm oalos Pericardialcavity fuea betweenvisceral mass and Protectsand housesheart hinge:dorsal to foot Heart Insideoericardial cavitv Pumosblood throushout bodv Kidneys brownishorgans found Wasteremoval flBf,p pericardial cavity Locate the mantle, a thin layer of tissue that covers the visceral massand foot. The visceral massis a soft mass of tissue located dorsal to the foot. The mantle is usually cream or yellow in color with a brownish edge in a preservedclam. Locate the incurrent siphon and excurrent siphon. The siphonsare folds in the mantle at the posterior of the clam. The incurrent siphon is ventral to the excurrent siphon. The incurrent siphon takes in water that contains oxygen and microscopic food particles. Water and waste materials are removed from the mantle cavity through the excurrent siphon. Examine thesestructures with a hand lens.

STOMACH INTESTINE

BUTBUSARTERIOSUS

POSTERIORFOOT RETRACTOR

POsTERIORADDUCTOR

ANUS

VISCERALGANGTION

EXCURRENTSIPHON ANTERIOR ADOUCIOR INCURRENISIPHON

CTENIDIUM(CUI) PEDATGANGLION

Labelthe drawingto the right with the following structures: anterioradductor muscle, posterioradductor muscle, incurrentsiphon, excurrent siphon, mantle, right valve

Tag eachof the following on your specimen:

Anterior adductormuscle, posterioradductor muscle, incurrentsiphon, excurrent siphon, mantle

Verified The diagram below shows the path of water through the body of the clam.

SUPRAERANCHIALCAVITY

MANTLE

POSTERIORRTIRACTOR MUSCLE

POSIERIORAODUCIOR ANI!RION FOOI REIRACION

INCURRENTSIPHON

ANI ADDUCIORMUSCI,I

<__--otREcTtoNoF FOOOMOVEMENT

With a pair of scissors,carefully cut awaya portionof the mantleas shownin the pictureto the right. (You might have alreadyperformed this stepwhen you openedthe clam and "tore" awaythe mantle.)

With the mantleremoved you cannow observethe gills, folds of tissuecovered with microscopiccilia. Gills arefound in pairs,one on eachside of the visceralmass. Use a probeand a handlens to examinethe gills. Observethe muscular,hatchet- shapedfoot locatedanterior and ventral to the gills. Locatethe palps,a pair of leaflikestructures ventral to the anterior adductormuscle and anterior to the gills. Themouth is a slit locatedbetween the palps,Water from the incurrentsiphon passes over the gills towardthe palps.Mucus and cilia on the palpstrap food anddirect it towardthe mouth.Water then circulates out of the mantlecavity through the excurrentsiphon.

What arethe two functionsof the eills? 1.

2.

Tag on your specimen: gill, foot Verified

I Locatethe visceralmass. In orderto studvthe visceralmass in detail,remove the gills andset them aside in your dissectingtray.

A. Thenuse a pair of scissorsto cut offthe ventralportion of the foot as shownin the diagramto the right. With a scalpel,carefully cut the remainingportion of the foot into right andleft sides.Now it is time to explore. Someof the organsbelow will be easyto find on your specimen andsome of themwill be very difficult. It dependson the quality of your specimenand the skill you impart. See whatyou canfind.....The diagram at thebottom of the pagemight provehelpful.

With the foot removed,locate the following structures: --reproductiveorgans, a spongyreddish mass; --the saclikestomach near the mouth; --the digestivegland, a light greenmass surrounding the stomach; --thecoiled intestine leading from the stomachto the anus nearthe excurrentsiphon; --thepericardial cavity, an areabetween the visceralmass andthe hinge; --theheart contained within the pericardialcavity; --thekidneys, spongy brownish organs below the pericardialcavity.

Pericardial sac Ventrlcfe(i) Ant€rior aorta Aurtctes(a

Digesflve gtands(2) Stomach Anterior Excurrent adductor siphon muscle Anus Mouth

Labial palps

Intestine Manile

t On the diagram below, label each of theseparts: visceral mass,reproductive organ, stomach,anus, digestive gland, intestine, anus,pericardial cavity, heart, and kidneys.

Follow your teacher'sdirections for storingthe clamfor furtheruse or properlydisposing of the clamand its parts.Thoroughly wash, dry, andput awayyour dissectingtray, scalpel,probe, scissors, and any other equipmentyou may haveused. Wash your handswith soapandwater.

Namethe threebasic types of mollusksand give an exampleof each(use your textbook). 1, n L,

a 1 How aremollusks classified (which phylum)? Whatstructures hold thetwo shellstogether in a bivalvemollusk? Whattype of feederis the clam? As watergoes through the clam'sbody, what doesit bring in? and As the water goesthrough the clam's body, what does it take out?

1 Reproduction Fertilizedeggs develoP Glochidiaare released intoglochidia in gill throughexcurrent siPhon of clamor mussel. Most musselsand clams are male or female; a few ra speciesare hermaphroditic.The reproductive cycle in o aa a theseorganisms is quite interestingin that the juvenile a stageis parasitic on fish. The eggsare releasedinto the cavity of the gills where fertilization takes place. Each zygote then developsinto alarva, called a glochidium. The larvae stay within the gill through the winter and are releasedinto the water the following spring. If they come into contact with a fish, a contact stimulus causes them to close their valves and thus becomeattached to the gills or the fins of the fish host.

What is the advantageof their being attachedto the gills Glochidiaare or fins? releasedfrom fish. (-, The tissueof the fish reactsby growingaround the .1. larva. After severalweeks, the parasiticlarval form is Larva become attachecl a releasedand begins a free-livingexistence. qP to gillsand fins aa of fish and a 69 become encysted. a Ad V^V t.

FlG.34.f Lifecycl€ of a clamor mussel.

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