Crayfish Dissection Lab

INTRODUCTION: If numbers alone are used as a measure of success, arthropods are the most successful animals on Earth. Their adaptations are more specialized, allowing them to survive in many areas. Sense organs, such as eyes and antennae, muscles arranged in groups, jointed legs, and a protective body covering have made it possible for these animals to become more successful than any other group of animals. Although jointed-legged animals vary greatly in structure, the basic body plan is the same. The crayfish is a large aquatic arthropod, which means it carries its skeleton on the outside. Because of its size, and representative anatomy of the jointed-legged animals, it is a useful dissection specimen. Please use this experience to study the general organization of the external anatomy of this group of animals. PURPOSE: To examine the crayfish as a representative example of the jointed-legged animals and to analyze the adaptations that allow it to thrive. STANDARDS: 1.4 Write in various modes in all content areas 1.5 Produce writing of high quality 3.1. C Explain the parts of a simple system and their relationship to each other. 3.3.A Describe the similarities and differences that characterize diverse living things. *How do animals adapt to their environment EXTERNAL ANATOMY PROCEDURE AND OBSERVATIONS: A. Put on goggles, a lab apron and gloves. Place the crayfish in the dissecting tray with its top side up. Like all crustaceans, a crayfish has an exoskeleton that covers its body. Look at the exoskeleton and find the two sections, the cephalothorax and the abdomen. The part of the exoskeleton that covers the cephalothorax is called the carapace. The abdomen is behind the cephalothorax and is also divided into sections. Count them. The abdomen ends in a segment called the telson. 1. How is the exoskeleton adapted to life on the bottom of the ocean? B. The carapace extends toward the front of the animal’s body and forms a hard beak called the rostrum. Find the compound eyes that lie beneath the rostrum. Examine the eyes with a hand lens and see if you can observe the many lens facets that make up a compound eye. Observe the diagrams and models of lens facets/compound eyes provided by your teacher. 2. What is the difference between a compound eye and your eye? C. The appendages closest to the front of the animal’s body are branched antennules. Locate the large antennae just behind antennules. The antennules are organs used for balance, touch, and taste. Long antennae are organs used for touch, taste, and smell. 3. What is the difference in structure of the antennae and antennules? 4. What is the difference in function of the antennae and antennules? D. Turn the crayfish over and examine the structures on the bottom of the animal’s body. Locate the mouth and the mandibles (jaws). Try to move the mandibles and see how they move in relationship to the crayfish’s mouth/face. The mandibles, or jaws, crush food by moving from side to side. 5. Describe the structure of the mandibles. 6. How does the structure of the crayfish mandible fit its function? E. Look near the mouth and find the three pairs of mouthparts known as maxillipeds. These are used during eating. Two pairs of maxillipeds hold solid food, tear it, and pass it to the mouth. The third pair of maxillipeds draws water over the gills. 7. How does the shape of the maxillipeds fit their function? 8. How does the shape of the walking legs fit their function? F. The next pair of appendages is the most obvious. These are claws. The chelipeds are used for defense and to capture prey. 9. How is the cheliped shaped to fit its function? G. The next four pairs of legs are called walking legs. Examine them carefully. H. Now, observe the appendages on the abdomen. We can use these appendages to determine the sex of your crayfish. The first five segments of the abdomen each have a pair of swimmerets, which create water currents and function in reproduction. All of the swimmerets in the female are small and soft. In the male, the first two pairs are hard and modified for transferring sperm. Now, find the place where the walking legs attach to the body. If you think you have a female, you can use a magnifying glass to find the crescent shaped slit that is the genital pore near where the 3rd pair of walking legs attaches. In a male, the sperm duct openings are found where fourth pair of walking legs attaches. After determining the sex of your specimen, be sure you have observed the opposite sex. I. At the back end of the animal’s abdomen is a pair of uropods, which together with the telson form the powerful tail fin, used in backward swimming. The crayfish moves backward by forcing water forward with this structure. Please note that the telson is an extension of the last abdominal segment, and not an appendage. Locate the anal opening on the bottom side of the telson. J. Now that you have examined the external anatomy of your crayfish, you will study the appendages, which the crayfish possesses. Observe all the appendages and compare them to the diagram attached to your lab. Be sure to look at the shape (structure) and relate it to the function of that appendage. Remember that the telson is not an appendage! 10. Overall, how does form fit function in the crayfish appendages? This is the end of the first day of the dissection. When you have reached this point, clean up your area thoroughly and work on the questions for this portion of the dissection. Do not go any further! INTERNAL ANATOMY PROCEDURE AND OBSERVATIONS: A. Put on glove, a lab apron, and safety glasses. Place the crayfish in the dissecting pan dorsal (top) side up. Carefully insert the point of the scissors under the top of the shell at the point indicated by your instructor and cut up the midline to the rostrum. Cut across the carapace just behind the eyes and remove the two pieces of the carapace. Be careful to remove the carapace slowly or you will rip some of the underlying tissue and may lose some organs that you must observe. Note the exposed gills. Each gill has blood vessels running through it and is feathery in nature, providing a large surface area for absorbing larger amounts of oxygen from the water. Move the walking legs and notice what happens to the gills. 11. Why are the gills branched? 12. What is the advantage of the gills being attached to the legs? B. Remove the gills from one side and carefully separate the muscles in the thorax to locate the light-colored heart just below. The crayfish has an open circulatory system in which the blood flows from arteries into sinuses or spaces in tissues. There are no veins. The blood flows over the gills before returning to the heart. Remove the heart and cut out the sides of the thorax to expose the organs underneath. The two lightcolored masses extending on each side of the body into the head are digestive glands. They produce an acid that is needed in the intestines to chemically break down food the crayfish eats. Most of the chemical digestion occurs once the acid and food reaches the intestine. 13.How do the digestive glands help in digestion? 14. Why is it good to have the digestive glands attached to the intestine? C. Between these digestive glands you should see a small pair of white reproductive organs in the male animal. The female will probably have a large mass of darkcolored eggs. These eggs must be removed before proceeding. To locate the intestine, insert the point of the scissors under the topside of the shell and cut back to the telson. Spread the shell, and the intestine will be found as a tube on the topside of the muscles of the abdomen. Trace it forward to the point where the intestine joins the large, thin-walled stomach. Cut into the stomach and examine the interior walls. Locate the hard, tooth-like structures which function in mechanical digestion of the food the crayfish eats. What does this mean? Where does this happen in your body? Your crayfish may also have food remaining in its stomach! 15. How do the hard, tooth-like structures aid the stomach in digestion? D. Now, remove all of the organs by cutting the short esophagus below the stomach and the bands of muscle holding the stomach just behind the eyes. You should be able to lift out most of the internal organs in one piece. Clean out the remaining tissue in the head so that the green glands are exposed just behind the antennules. These structures function like your kidneys. What does this mean that they do for the crayfish? 16. What color are the green glands? (They are NOT green.) 17. What is the function of the green glands? E. In the front part of the head cavity, between the eyes is a small mass of white tissue, the brain. Trace the nerves that go from the antennae and the eyes. Then trace the nerve cord back from the brain to the abdomen. The enlargements of the nerve cord in each segment of the abdomen are the ganglia. You should be able to find the nerve cord that runs on the ventral (bottom) side of the crayfish. The crayfish uses its abdomen (tail) for defense and can curl up or use the tail to quickly swim away if threatened. 18.Why is the crayfish’s nerve cord on the bottom of its body? .