15. Explain the function of a swim bladder. 16. Explain circulation in a bony fish. 17. Explain digestion in bony fish. 18. What is the function of the operculum? 19. What is ectothermy? 20. What is parthenogenesis? 21. Name the three structures that amphibians can use to respire throughout their lives.

15. The swim bladder is a gas-filled sac located in the dorsal portion of the body cavity of most bony fishes. Its primary function is to control buoyancy. By regulating the amount of gas in the swim bladder, a fish can adjust its density to match the density of the surrounding water, allowing it to remain at a specific depth with minimal effort. This conserves energy that would otherwise be expended on swimming to maintain vertical position.<br /><br />16. Circulation in bony fish follows a single-loop pathway. Deoxygenated blood from the body enters the sinus venosus, then the atrium, followed by the ventricle, a powerful muscular chamber that pumps the blood to the gills. In the gills, thin filaments lined with capillaries facilitate gas exchange. Oxygen is absorbed from the water, and carbon dioxide is released. The now oxygenated blood flows from the gills to the rest of the body, delivering oxygen to tissues and organs before returning to the heart. This is known as a single circulatory system because the blood only passes through the heart once in each complete circuit.<br /><br />17. Digestion in bony fish begins in the mouth, where teeth are used to capture and hold prey. Some species also have pharyngeal teeth located in the throat. Food then passes through the esophagus to the stomach, where it is further broken down by digestive enzymes and acids. From the stomach, food moves into the intestine, where nutrients are absorbed. The liver and pancreas secrete digestive enzymes into the intestine to aid in this process. Undigested material is expelled through the anus.<br /><br />18. The operculum is a bony flap that covers and protects the gills of bony fish. It plays a crucial role in respiration by aiding in the movement of water over the gills. The operculum opens and closes, creating a pressure gradient that draws water in through the mouth and over the gills, where gas exchange occurs.<br /><br />19. Ectothermy is a thermoregulatory strategy in which an animal's body temperature is primarily determined by the external environment. Ectotherms, like fish, amphibians, and reptiles, rely on external sources of heat, such as sunlight or warm surfaces, to regulate their body temperature. They do not generate enough internal heat to maintain a constant body temperature.<br /><br />20. Parthenogenesis is a form of asexual reproduction in which an egg develops into an embryo without being fertilized by sperm. It occurs in a variety of organisms, including some fish, amphibians, reptiles, and insects. Offspring produced through parthenogenesis are typically genetically identical to the mother.<br /><br />21. Amphibians can use three structures for respiration throughout their lives (or at different stages):<br /> * **Gills:** Primarily used in the larval (tadpole) stage of aquatic amphibians.<br /> * **Lungs:** Develop as the amphibian matures and transitions to a terrestrial or semi-terrestrial lifestyle.<br /> * **Skin:** Amphibians can absorb oxygen and release carbon dioxide through their moist, permeable skin. This cutaneous respiration supplements lung function and is sometimes the primary method of gas exchange in some species.<br />