Biology Dissection II THE RAT Name___________________ Note: The organism you are cutting up today was once alive and vibrant, a living being. Dissection of a complex organism is not a right, but a privilege. Please be aware that this privilege is one that many students do not have. Background Rodents (from the Latin rodere, “to gnaw”) are the largest order of mammals, comprising about 40% of the total mammalian species. All mammals, including rats and humans, share some common characteristics: A. B. C. D. E. Body covered with hair, but reduced in some Circulatory system with a four-chambered heart Homoeothermic (warm blooded) Internal fertilization Young nourished by milk from mammary glands For many reasons rats are a much maligned group of animals. They carry and spread disease (the Bubonic Plague or “Black Death” of the middle 14th century was spread by rats), and their toughness, intelligence and tenacity, especially in large numbers, make them extremely difficult to deal with. Part I – External Anatomy Briefly study the external anatomy. Locate the ears, whiskers, nostrils, and anus, which is beneath the tail. What is/are the function(s) of the rat’s tail? Notice the claws and sharp teeth (incisors). Determine whether your rat is a male or female. Male rats are larger, and will have testicles locate ventral to the anus. Before proceeding, record the following information, to be used in your lab report: Based on your examination of the rat’s external anatomy, identify three features that you could use to recognize a mammal. Is this a placental, marsupial, or monotreme mammal? How can you tell? What can you tell about this animal’s lifestyle? Where does it live? What sort of things does it eat? How is it adapted for its niche in the environment? Name some related animals – ones that are likely to be in the same family. Part II – Internal Anatomy Incisions 1. To make the first cut (lengthwise) pinch some skin together on the abdomen and use the scalpel (JUST THIS ONE TIME) to open the flesh. 2. Use the scissors to continue the cut as shown. Just as in the frog, there are two layers you need to get through, skin and muscle. Digestive System Warning: The Rat’s body has TWO cavities (the Thoracic and Gastrovascular), separated by the diaphragm muscle. Be careful not to damage the diaphragm or the organs of each cavity. You may have to flush out your rat’s abdomen under flowing water in the sink to remove the fluid in the gastrovascular cavity. The abdominal organs may still be covered with a membrane, the peritoneum, (peritoneal membrane) but this usually comes off with the overlying layers. If necessary drain the body cavity of excess liquids. Another membrane, the mesentery, surrounds and supports most of the digestive system and its related vasculature (blood vessels), and in human males is a primary storage site for fat, causing "beer bellies" in some men. 1. Locate the large, multi-lobed liver. How many lobes does the rat liver have? (How many lobes did the fog liver have?) Don't count the long, narrow spleen, which is lateral to the stomach on the left side. (The spleen is not a digestive organ, but rather a major storage site for oxygen-carrying red blood cells, and immune-system white blood cells.) Also locate the diaphragm muscle, which is immediately anterior to the liver. Notice how the liver is fed by blood vessels that pass through the diaphragm. What other tubes pass through the diaphragm? The diaphragm muscle in humans 2. Locate the stomach, which is in a similar location on the left as it is the frog. The pylorus is a muscle in the stomach found at the border of the stomach and the esophagus, which is continuous with the rat’s mouth. Cut out the stomach neatly and inspect its inner surface. Locate the pyloric sphincter, a muscular valve, at the posterior end of the stomach. This valve mediates passage of food into the small intestine. 3. The stomach joins the small intestine via the duodenum. Notice that the small intestine is actually quite long, and is coiled many times. What is the name of the tissue that keeps all the coils and the associated blood vessels in place? The small intestine is continuous with the large intestine. Why is the large intestine called “large” if the small intestine is so much longer? 4. Locate the pancreas within the mesentery. The pancreas controls blood sugar levels with hormone commands to the liver. Once you feel confident with the organs of the digestive system remove only the stomach and intestines CAREFULLY. If you have not located each of the organs above, do not continue to the next section! Before continuing, also note for your report: Why is the large intestine called “large” if the small intestine is so much longer? Based on what you have seen so far, what adaptations does this animal have to its lifestyle? Think of teeth and intestines, in particular. Do the best that you can to identify the sexual organs. Females In females, the ovaries, which produce egg cells and are small and somewhat peanut-shaped, located just posterior to the kidneys. The oviducts (AKA Cornuras, AKA Uterine horns) lead from the ovaries to the Yshaped uterus, which connects by way of the vagina to the urogenital opening. Males In males, testes, which produce sperm and male hormones, start out up inside the body cavity during fetal development, then migrate out through two canals into the scrotum. Attached to each testis is an epididymis, which stores sperm and leads to the vas deferens, a tube that attaches to the urethra, which carries sperm out of the body through the penis. Male Female If you have not located each of the organs above, do not continue to the next section! Before proceeding, note the following: Which structures in the male and female do you think are homologous? In other words, which structures develop from the same precursor in the embryo? What do you think the kidneys do? How would the kidneys need to be modified if the rat lived in water instead of on land? The Thoracic Cavity – The Heart and Lungs 1. The organs of the thoracic cavity are covered by various thin membranes. These lubricated surfaces allow the two sets of organs to move vigorously without wearing each other away. The heart is covered by the pericardium. 2. Locate the diaphragm, which is the posterior wall of the thoracic cavity. As shown in pictures on earlier pages, the contraction of this muscle increases the space of the chest cavity; the lungs fill with air to fill that extra space. 3. Locate the lungs and count each of their lobes. Does one lung have more lobes than the other? Remove the tissue around the heart and identify the following chambers. Right atrium – receives blood from the superior and inferior vena cavas, the major veins bringing deoxygenated (though nutrient rich) blood from the body. (FIND THEM!) Left atrium – receives freshly oxygenated blood from the lungs via the pulmonary vein(s) Left Ventricle – sends oxygenated blood out the ascending (going up) and then descending (going down) aorta to the body 5. Note the network of blood vessels across the surface of the heart - these coronary arteries branch off the aorta to feed the heart muscle, which has huge oxygen requirements! If these small vessels clog up, the parts of the heart they would ordinarily be feeding seize up, causing a heart attack. Lab Report: In your lab report, type out the answers to the questions/observations (STOP SIGNS) you noted during the dissection. You do not need to do additional research to answer these questions – they should be based solely on your observations during dissection and your intuition and prior knowledge. Then answer the following questions, which you might need your book and the following website: http://www.mnh.si.edu/mammals/ 1. Describe several key distinctive features of mammals. (Look at the WHAT MAKES A MAMMAL section of the website) 2. Explain how the three major types of mammals (placental, marsupial, and monotreme) differ in terms of reproduction and anatomy (read section 19.8 in the textbook) 3. How are mammals similar to other land vertebrates? 4. Describe how mammals have changed over time. (Look at the HOW DID MAMMALS EVOLVE section of the website) (DRAG THE SLIDER ON THE TIMELINE to see the information change) Read each time period on the timeline and then in your own words, summarize the overall evolution of mammals. 5. On the same website, look at the 65 MYA time period. Under the “Mammal Spotlight” pane click ZALAMBDOLESTES. Compare this ancient mammal to the rat. What features do they have in common? How are they different?