Fishes+and+Amphibians

Fish Structure of Fishes Fishes can be classified into four classes of subphylum vertebrata. Fish breathe using gills and have a two chambered heart. They also reproduce sexually. Most fishes have paired fins, which are fan shaped membranes that are used for balance, swimming and steering. All fishes have highly developed sensory systems. Cartilagionous and bony fishes have an adaptation called the lateral line system that enables them to sense objects and changes in their environment. The lateral line system is a line of fluid-filled canals running along the sides of a fish that enable it to detect movement and vibrations in the water. Most fishes have scales that are thin bony plates formed from the skin. The evolutions of jaws in fish have allowed the fish to have the ability to grasp and crush their prey with great force. The evolution of bones in some fishes have allowed them to adapt to a variety of aquatic environments. Bony fish have evolved swim bladders. They are a thin-walled, internal sac found just below the backbone in most bony fishes. Origins of Fishes Scientists have identifid fossils of fishes that existed during the late cambrian period, 500 million years ago. Ostracoderms were early jawless fish that became extinct at the end of the Devonian Period, about 354 million years ago. Scientists hypothesize that the jawless astracoderms were the common ancestors of all fishes. Amphibians Structure of Amphibians The class Amphibia has three orders: Cudata Salamanders and newts, Anura-Frogs and Toads, and Apoda-legless caecilians. Amphibians have thin, moist skin and most have four legs. Most rely on water for reproduction. Amphibians are more common in regions that have warm temperatures all year because they are ectotherms. An ectotherms is an animal that has a variable body temperature and gets its heat from external sources. Unlike fishes, most amphibians go through the process of metamorphosis. Fertilized eggs hatch into tadpoles, the aquatic stage of most amphibians. As tadpoles grow into adult frogs and toads, they develop legs, lungs, and a three-chambered heart. Origins of Amphibians One type of tetrapod had evolved that retained gills for breathing and a finned tail for swimming. Early tetrapods may have used their limbs to move on the bottom of marshlands filled with plants. In later fossils, the four limbs are found further below the body to lift it off the ground. Most likely, amphibians arose as their ability to breathe air through well developed lungs evolved. The success of inhabiting the land depended on adaptations that would provide support, protect membranes involved in respiration, and provide efficient circulation.