Observe a fish dissection, discuss adaptations and functions, complete worksheets.
Skills:
Observing, listening, reporting, discussing, comparing, identifying, labeling.
Objectives:
1. Name external and internal parts of a fish and identify their functions.
2. Explain how fish are specially designed for their environment.
3. Contrast and compare a fish's body parts to a human's.
Materials:
Preparation:
1. If you cannot find a whole trout or salmon, use some other kind of fish. If you have a
freshly caught fish, it may be frozen until you're ready to dissect it. Consider cooking
and eating the fish after dissection if it is large enough.
2. Choose a location where all the students can see clearly. Make allowances for those students who may feel uneasy about dissection.
3. Cover the work surface with newspaper and paper towels. Keep sharp instruments out of children's reach.
Procedure:
Explain the function of each feature as you go along.
External features:
1. Let students feel the fish's skin. Discuss the purpose of the slime. (It protects
against growth of fungus and it helps fish slide through water). Use a magnifying glass to
see how the scales are arranged. Why do fish have scales?
2. Look at the color pattern. Discuss the advantage of being dark on top and light on the belly. Why does the fish have spots? Does the fish have the same markings all its life? Why? (Stream and ocean environments require different protective coloration or camouflage).
3. Observe the lateral line. Discuss what it is used for and the way it works.
4. Discuss the overall shape of the fish and how this is an advantage. Look at placement of the fins and ask students to imagine a fish swimming in the water. How does it move? How are the fins used? Note the range of movement of each fin. Pectorals can rotate 180 degrees, other fins are less flexible.
5. Feel the bony rays that support the fins. Count the number of rays on the anal fin. This is one of the distinguishing characteristics among salmonids.
6. Note the size of the eye. Its relatively large size, and the large pupil, tells us how important vision is for this animal. Look for eyelids (there are none). Observe the tough, clear membrane that covers the eye. Rotate the eye in its socket with your finger.
7. Locate the nostrils. Describe the large olfactory lobes that are located in the brain. Why are salmonid's smell receptors so highly developed?
8. Open the mouth and look at the color of the gums and surrounding area . Feel the teeth along the gum margins and on the roof of the mouth. Do fish chew? (Teeth are used for grasping and holding prey.) Find the teeth on the tongue, which help get food down. Does the tongue feel like a human tongue? See how wide the mouth can open. Why is this?
9, The mouth is also used for breathing. In low oxygen conditions, fish can actively pump water over their gills by opening and closing their mouth. Try this, relating it to the action of a pump.
10. The gills arches can be seen by looking down the fish's mouth. Use a probe to separate the arches and explore how they are arranged.
11. Place the fish on its side and look at the operculum, the bony plates which protect the gills. Lift the operculum and look at the gills. Now cut the operculum away at its base, exposing the gills.
12. Remove the gills by cutting the upper and lower attachments of the arch. Look at the gill rakers, the bony projections along the inside curve of the arches. Observe the large surface area provided by the gill filaments, and the thin tissue which allows blood vessels to come into contact with the oxygen in the water. Compare and contrast gills and lungs.
Internal Organs:
13. Carefully cut the fish open using scissors or a scalpel (up from the anus to the
bottom of the jaw, taking care not to cut into the fish's internal organs). Before moving
any organs, let students see how everything fits together. Look for the thin, transparent
membrane that encloses the organs. Cut away the flap of skin and look for fat deposits,
which are found around the stomach. Discuss the importance of fat reserves for migrating
fish that do not eat once entering fresh water.
14. Look for the swim bladder. It is made of very thin tissue and is located in the upper body cavity, below the kidneys. It will be less well developed in small fish and since it will not be inflated you may not see it. If you cannot find it, point to its location and discuss its function.
15. The male reproductive organs will be flaccid white or orange tissue near the intestines. Eggs may or may not be noticeable in females. Both will vary in size depending on maturity of the fish.
16. Put the fish on its back and find the kidneys, located just under the backbone. They are thin, dark in color, and run the whole length of the body cavity. Discuss their functions (important for salmonids living in both fresh and saltwater).
17. Investigate the digestive track by starting in the mouth and following the route that food would take. Put the probe through the mouth and into the esophagus to show the beginning of the route. Then follow the course of the stomach using your finger or the probe. The first area of the stomach is called the cardiac stomach; this is where digestion begins. Notice the different kinds of tissue that make up the stomach.
18. The pyloric stomach is that portion from which the pyloric ceca project. It begins at the bend below the cardiac stomach, and is made of different tissue. Discuss how the stomach area is increased by the pyloric ceca. How does this improve the function of the stomach?
19. The intestines provide the last chance to extract nutrients from food. Notice the network of blood vessels which are used for nutrient exchange. Follow the intestines to the anal opening where waste products are eliminated.
20. The spleen will be seen by lifting the stomach. It is a reddish organ found at the end of the cardiac stomach. Discuss its function.
21. The liver is just in front of the stomach. Talk about the liver's role in digestion of fats. The gall bladder is a mass of darker tissue on the liver.
22. A fish's heart is almost literally in its mouth. It can be found by moving the liver. You should be able to make out the different chambers. The fact that the gills, heart and liver are so close together is no coincidence. Blood pressure is best near the pump (heart). Blood is filtered by the liver, and absorbs oxygen from the gills; both are vital functions.
23. Cut through the fish to expose the back bone and muscles. Observe arrangement of the muscle masses. (This is the part of the fish we eat.)
24. Carefully cut away the skin by lifting it while running the scalpel along the skin-muscle interface. If you have a small fish, the skin may be thin enough to place under a microscope. If so, observe the pattern of the scales, the growth rings on the scales, and the darkly pigmented spots. If the skin is too thick, use a magnifying glass. Try to remove some of the scales so you can look at the rings of growth.