As you discuss each system and the variations in nature, these are the three concepts you will want to emphasis. Let the students tell you the significance of each of these.
Have the students point out that the specific structure of an object determines its function. Here you can compare the structure of Velcro with the structure of a cocklebur. George de Mestral was hiking when he noticed cockleburs sticking to his clothes and his dog’s fur. He went home and looked at a burr under a microscope. He noticed tiny hooks on the burs that grabbed onto the threads of his pants. From this he invented Velcro.
Have the students identify systems they think function using diffusion. Let them tell you how this would work in each system. The systems you would want to stress are: digestive, circulatory, respiratory, and excretory.
Have the students explain the relationship between villi and the terry cloth. In which systems would surface area be so important. The systems would be digestive, respiratory, excretory, circulatory, integumentary and nervous.
Let the students combine all the concepts of surface area, diffusion and structure and function to discuss how the villi function. The structure of the villi gives a lot of surface area so the function of diffusion will have lots of surface area on which to work.
The students can now refer to the chart on comparative anatomy. You will want to point out the differences in each system. We do not want the students to know the phyla or the specific parts of each digestive system. We want them to see the concepts of diffusion, surface area and structure and function with each type of digestive system. The Cnidarians only have one digestive opening and a large gastrovascular cavity. They rely on diffusion in that cavity to take nutrients to their cells. The annelids have the long tube with two digestive openings that allow more surface area for diffusion. The Arthropod’s long tube gives more surface area. The Arthropods also have two digestive openings. Finally, the mammal intestine has many folds. Internally there are the villi that add even more surface area for diffusion. Mammals also have two digestive openings. Point out that the structures are suited for the function.
Point out that Cnidarians get oxygen by diffusion from the water. Annelids get oxygen by diffusion through the skin. Book lungs have layers of respiratory tissue stacked like the pages of a book. This gives more surface area so diffusion can occur. Gills are featherlike organs that specialize in the exchange of gases from water. The structure gives increased surface area for the diffusion to occur. The alveoli in the mammal lungs are tiny air sacs that provide increased surface area for gas exchange. The surface area of the 300 million alveoli (each lung has 150 million alveoli) in the human lungs would cover 70 square meters. That’s about the size of an average classroom measuring 25 feet by 25 feet.
Cnidarians have no circulatory system or blood. Nutrients diffuse from cell to cell. Annelids have a closed circulatory system and 5 pair of aortic arches that pumps the blood. Their blood vessel system is also primitive. Explain what a closed system is. Arthropods have an open circulatory system where the blood is pumped by a heart into the body cavity. The blood surrounds the organs so that diffusion can occur. The mammal system is closed and has a muscular heart that pumps blood through a complex system of vessels. All of these systems rely on diffusion of substances from the blood into the cells. The more blood vessels there are, the more surface area there is for diffusion to occur.
Cnidarian excrete their waste from their cells by diffusion into the surrounding water. Annelids have developed a series of tubes that filter liquid waste from the coelom (body cavity) then release the waste into the environment. Terrestrial arthropods have a saclike organ that extracts waste from blood.
Mammals have a complex tube system that extracts waste from blood. All of these systems rely on diffusion of the waste. Each one has developed a specific structure to accomplish this. The mammal excretory system has developed in such a way as to increase the surface area so the diffusion process is more efficient.
The function of the nervous system is to detect stimuli from the environment. The Cnidarians have developed a nerve net that is a network of nerve cells. There is no brain. The annelids have developed a primitive brain with 2 nerve cords (a dorsal and ventral). Many have variations of sense organs. Arthropods have a brain and a ventral nerve cord. They have ganglia coming off of the nerve cord that send messages to their appendages. Most arthropods have well-developed sense organs. The mammal nervous system has a well- developed brain. The many folds in the brain give the brain more surface area. The nervous system is a complex system of neurons that detects stimuli from the environment and initiates a response.
Skeletons protect and support the body. An exoskeleton is an external skeleton that protects and supports the body. Many exoskeleton have a waxy covering that helps prevent the loss of water. An endoskeleton is an internal skeleton that supports the body, provides protection for internal organs, assists in movement, stores minerals, and provides the site for blood cell formation.
