9.2: Procedure

Dissect Preserved Mussel Specimen

Follow the instructions outlined below to carefully complete the dissection of your specimen:

1. Wear gloves and eye protection. Obtain a dissecting tray, dissecting kit, dissecting pins, and a mussel from the specimens provided. Also obtain a complete mussel dissection guide which includes images that you can reference to help you with your dissection.
2. Place the mussel in the dissecting tray with the hinge (dorsal side) up. Identify anterior/posterior openings by the longer/shorter ends: the anterior end is usually toward the siphons. Create a sketch of the external surface of the mussel. Be sure to measure the dimensions of the shell and record that in the space provided.
3. Stabilize the mussel in the tray with the dorsal hinge toward you. Insert the tip of a blunt probe or small flat screwdriver into the posterior shell gape (the wider opening at the back where the siphons are). Do not force but instead work gently to pry the valves slightly apart. Gently slide the scalpel along the opening to sever the adductor muscles where they attach to the shell. Cut close to the muscle rather than through the tissue, if possible, to preserve internal contents. Once both adductor muscles are severed, gently open the mussel like a book.
4. Gently separate the mantle lobes at the posterior end to reveal the mantle cavity and gills. Note any incurrent/excurrent siphonal openings.
5. Gently lift a gill plate with forceps. Note the gills are often in pairs folded in a fan-like arrangement.
6. Carefully separate the mantle from the visceral mass using the probe from your dissecting tray, taking care to loosen the tissue. You may need to cut small connective tissues to lift the visceral mass slightly.
7. Find the muscular foot, which is usually located in the anterior region. Gently tease it free from surrounding tissue to observe muscle fibers.

Observe the Dissected Mussel

Carefully observe the dissected mussel, being sure to note and label the characteristics below. Record your sketches and observations in Data Table 9.1.

1. Shell: hard, protective outer coat; in this case there are 2 similar shells enclosing the entire animal.
2. Mantle: tissue lining the shell; surrounds the visceral mass and produces shell material.
3. Visceral mass: central soft region encased within the mantle; contains digestive, reproductive and excretory organs such as nephridia.
4. Gills aka ctenidia: plate-like structures with many folds, located on either side of the visceral mass; used for respiration and filter feeding.
5. Muscular Foot: large, muscular organ used for burrowing; very fibrous.

Dissect Preserved Earthworm Specimen

Follow the instructions outlined below to carefully complete the dissection of your specimen:

1. Wear gloves and eye protection. Obtain a dissecting tray, dissecting kit, dissecting pins, and an earthworm from the specimens provided. Also obtain a complete earthworm dissection guide which includes images that you can reference to help you with your dissection.
2. Place the worm in dissecting tray on a paper towel and blot excess preservative. Measure total length and note approximate number of visible segments. Create a sketch of the animal and record this information in the space provided.
3. Distinguish the anterior (head) vs. posterior (tail) ends. The anterior end is typically more tapered and contains the mouth; the posterior end is blunter. Locate and note the clitellum present in the anterior third of the animal’s body; this is a thickened band used for reproduction. Add this structure to your sketch if you have not done so already and take note of its segmental position.
4. Examine individual segments with a magnifying glass or by placing them under dissecting microscope. Note the positions of chaetae/setae and the tiny pores of nephridia if visible.
5. Place the worm dorsal (belly) side up in the dissection tray. Gently stretch the worm straight (do not overextend) and insert pins through the body wall at several segments near each end and one or two along the mid-body to stabilize. Pin at an angle to secure the specimen.
6. Make note of the dorsal midline. Carefully, make an incision along this midline. To do this, use scissors or a scalpel to make a small transverse nick at the posterior pinned end (careful not to cut deep). Next, with the scissors, carefully cut along the dorsal midline from posterior toward anterior through the dorsal body wall only, cutting segment by segment. Keep the blade shallow and controlled — you want to open the coelom without severing internal organs. Stop just before getting to the anterior/head region to avoid cutting into the pharynx.
7. Gently fold back and pin the two dorsal body flaps (cut edges) to expose the internal cavity. Place pins in alternate segments to hold flaps open without tearing tissue.

Observe the Dissected Earthworm Specimen

Carefully observe the dissected earthworm, being sure to note and label the characteristics below. Record your sketches and observations in Data Table 9.2.

1. Pharynx (segments ~1–6): just posterior to the mouth find the muscular pharynx — thick, muscular chamber used to ingest soil. It may be pale, rounded. Gently move aside mantle tissue to view.
2. Esophagus (segments ~7–9): a short tube posterior to the pharynx. Several muscular aortic arches/hearts encircle the esophagus near the anterior region. They can be distinguished as small, rounded muscular structures.
3. Crop (food storage; variable segments): posterior to the esophagus; a thin-walled storage chamber. It appears as an expanded region of the gut.
4. Gizzard (muscular grinding organ): posterior to the crop; thicker muscular wall used to grind soil and food. Palpate gently with probe to appreciate thickness.
5. Intestine: follows the gizzard; a tube-like structure that runs the remainder of the body.
6. Nephridia (excretory tubules): found in most segments in pairs; look for thin, coiled tubular structures along the body wall/near septa. You will need to place the specimen under the dissecting microscope to view effectively.
7. Septa: internal septa separate segments and the body wall.
8. Chaetae: bristle-like structures; located on the underside and sides of each body segment, except for the first and last ones. You will need to place the specimen under the dissecting microscope to view effectively.

   Clean-Up

   Use the following instructions to clean up the dissected specimen, only after other groups have had a chance to observe the specimen:

   1. Carefully unpin and remove specimen pieces. Dispose of biological waste per instructions (biohazard bag or classroom trash as directed). Rinse and disinfect dissecting instruments making sure they are dry prior to returning to the dissecting kit. Use caution as these tools can be sharp!
   2. Also rinse your dissecting tray, leaving it to air dry next to the sink. Next, clean the lab bench of your workspace. Wash hands thoroughly after removing gloves.

Dissect Preserved Crayfish Specimen

Follow the instructions outlined below to carefully complete the dissection of your specimen:

1. Wear gloves and eye protection. Obtain a dissecting tray, dissecting kit, dissecting pins, and a crayfish from the specimens provided. Also obtain a complete crayfish dissection guide which includes images that you can reference to help you with your dissection.
2. Place the crayfish dorsal side up in dissecting tray on a paper towel and blot excess preservative. Measure body length and observe external features. Create a sketch of the animal and record this information in the space provided.
3. Distinguish the anterior (head) vs. posterior (tail) ends. The anterior end is where you will find several sensory structures such as the eyes and mouthparts. Locate and note the carapace which is a thickened region that covers the cephalothorax. Stretch gently and pin the legs and tail to secure the specimen. Use pins at both the anterior and posterior end of the animal to stabilize.
4. Locate the seam of the dorsal carapace. Using small scissors or a scalpel, make a shallow cut through the carapace. Be cautious, making sure to cut only the shell, not the underlying tissue.
5. Carefully lift and remove the dorsal carapace to reveal the internal cephalothorax cavity. Set the removed piece aside. Pin the tissue back if needed to hold the cavity open.
6. Identify the gills (paired comb-like structures) and carefully remove them with forceps and transfer them to a dish containing saline. Observe the gills under the dissecting microscope.
7. The heart sits dorsally within the pericardial sinus, just posterior to the gastric region, often faintly muscular and may be visible as a triangular structure.
8. Turn the specimen ventral side up (carefully unpin and re-pin with ventral side facing up). Pin legs laterally so abdomen is flat.
9. Make a mid-ventral incision along the abdomen between the walking legs and continue along the rest of the body: using fine scissors, cut through the exoskeleton on the ventral abdomen along the midline down to but not through the intestine. Cut segment by segment. Be careful, cutting too deep may sever the gut.
10. Trace the intestine from the cephalothorax through the abdomen to the anus.
11. Locate and identify the ventral nerve cord which consists of two nerve cords fused into a double chain of ganglia running ventrally along the midline. The paired segmental ganglia may be visible as whitish nodules.

Observe Preserved Crayfish Specimen

Carefully observe the dissected crayfish, being sure to note and label the characteristics below. Record your sketches and observations in Data Table 9.3.

1. Exoskeleton: hard, outer covering that surrounds the entire animal’s body.
2. Carapace: dorsal, anterior part of exoskeleton observed as a thickened band in the cephalothorax region.
3. Tagmata: note the cephalothorax (a fused head and thorax) and the abdomen (the segmented tail) that represent the functional regions of the animal’s body.
4. Jointed appendages: these are the paired appendages that include:
   • Short, sensory structures (antennules) and long, sensory structures (antennae) in the anterior most region.
   • Mouthparts (mandible, maxilla and maxillipeds) near the mouth.
   • Walking legs with claws (chelipeds) and similar sized walking legs (pereiopods) attached laterally to the cephalothorax region.
   • Uropods and telson forming the outside and inside of the tail fan, respectively.
5. Gills: Feather, like structures located just underneath the carapace in the cephalothorax region.

   Clean-Up

   Use the following instructions to clean up the dissected specimen, only after other groups have had a chance to observe the specimen:

   1. Carefully unpin and remove specimen pieces. Dispose of biological waste per instructions (biohazard bag or classroom trash as directed). Rinse and disinfect dissecting instruments making sure they are dry prior to returning to the dissecting kit. Use caution as these tools can be sharp!
   2. Also rinse your dissecting tray, leaving it to air dry next to the sink. Next, clean the lab bench of your workspace. Wash hands thoroughly after removing gloves.

Dissect Preserved Sea Star Specimen

1. Follow the instructions outlined below to carefully complete the dissection of your specimen:
2. Wear gloves and eye protection. Obtain a dissecting tray, dissecting kit, dissecting pins, and a sea star from the specimens provided. Also obtain a complete sea star dissection guide which includes images that you can reference to help you with your dissection.
3. Place the sea star dorsal side up in dissecting tray on a paper towel and blot excess preservative. Measure overall diameter from arm tip to opposite arm tip and observe external features including the madreporite. Create a sketch of the animal and record this information in the space provided.
4. Turn the specimen ventral side up and observe the oral surface with the mouth at the center and the groove running down each arm.
5. Using small scissors or a scalpel, make a shallow incision in one arm along the center of the groove from about mid-arm toward the disc at the center of the animal. Be sure to keep the cut shallow so that you do not disturb the internal organs.
6. Gently use forceps or a blunt probe to separate the cut edges and expose the internal cavity of the arm. Pin the edges open laterally to hold the incision and reveal internal structures. If you are observing a mature specimen, you will find prominent gonads throughout each arm. If present, remove the gonads to reveal the underlying structures.
7. Examine the arm under the dissecting scope to observe the tube feet (podia) and the calcified plates (ossicles) of the endoskeleton.
8. Make a short incision at the edge of the central disc (near the mouth) to extend exposure into the central disk. Be cautious around the mouth and ring canal. Take note of the ring canal and its attachment to the radial canals in each arm as well as to the stone canal which leads to the madreporite. You may need to observe using the dissecting microscope for a clearer view of internal features.

Observe Preserved Sea Star Specimen

Carefully observe the dissected sea star, being sure to note and label the characteristics below. Record your sketches and observations in Data Table 9.4.

1. Madreporite: sieve-like plate found on the dorsal side near the center of the animal’s body.
2. Stone canal: small, tube-like structure which connects the madreporite to the ring canal.
3. Ring canal: circular, tube-like structure found in the center of the animal’s body.
4. Radial canals: narrow, tube-like structures that extend throughout each of the animal’s arms.
5. Ampullae: muscular bulb-like structures which connect the tube feet to the radial canals.
6. Tube feet/podia: small, sucker-like structures which are attached to the radial canals and extend through the groove of each arm.
7. Endoskeleton: small overlapping bony plates known as ossicles which create the internal structure of the animal’s body.

   Clean-Up

   Use the following instructions to clean up the dissected specimen, only after other groups have had a chance to observe the specimen:

   1. Carefully unpin and remove specimen pieces. Dispose of biological waste per instructions (biohazard bag or classroom trash as directed). Rinse and disinfect dissecting instruments making sure they are dry prior to returning to the dissecting kit. Use caution as these tools can be sharp!
   2. Also rinse your dissecting tray, leaving it to air dry next to the sink. Next, clean the lab bench of your workspace. Wash hands thoroughly after removing gloves.