I. General Characteristics.

    A. “Double Life” – water to land.
    B. Respiration through gills, lungs, skin, and mouth.
    C. 3-chambered heart.
    D. Ectothermic.

II. Orders.

    A. Apoda: caecilians.
        1. Small and wormlike: no limbs.
        2. Most are subterranean.
        3. Most are blind as adults.
        4. Burrow to find worms and invertebrates to eat.

    B. Caudata: salamanders.
        1. Both pairs of limbs are identical.
        2. Most are small, 6-10 in.
        3. Mud puppies may reach 2 ft.

    C. Anura: frogs, toads.
        1. No tails.
        2. Powerful hind limbs for jumping and swimming.
        3. Small front limbs prop up the body.
        4. Webbed hind feet for swimming.

III. Life Cycle.

    A. Jelly-like eggs laid in water
    B. Male fertilizes eggs by discharging milt over them.
    C. Aquatic larva hatches.
        1. Frog larva: tadpole – looks like a fish.
        2. Salamander larva: looks like adult, but with external gills.
    D. Metamorphosis.
        1. Tadpole.
            a. Gills degenerate.
            b. Lungs form
            c. Legs grow out.
            d. Eyes grow and bulge.
            e. Mouth widens.
            f. Tail is absorbed.
        2. Some salamanders change into land-dwellers only when their watery habitat dries up.
        3. The mud puppy remains aquatic throughout its life.
    E. Adult resides on land.
        1. Hibernation during winter.
        2. Estivation in summer.

IV. Skin.

    A. Smooth, scaleless, moist.
    B. Abundance of blood vessels.
    C. Secretions, sometimes poisonous.
    D. Color.
        1. Provided by chromatophores.
        2. Camouflage for some.
        3. Warning of poison on skin for others.

V. Respiration.

    A. Gills.
        1. Common to all amphibians.
        2. Degenerate when the animal moves to land.

    B. Lungs: not in all.
        1. Frog lacks rib-diaphragm complex.
        2. Lowers floor of mouth, drawing in air.
        3. Closes nostrils and swallows air, forcing it into lungs.
        4. Contracts body wall to expel air.

    C. Lining of mouth and throat.

    D. Skin.
        1. 90-95% of the respiration of some lungless salamanders.
        2. Frogs use it when submerged for a long time and during hibernation.

VI. Circulation.

    A. Deoxygenated blood returns from the body through the 3 venae cavae.
    B. The venae cavae empty into the sinus venosus, a sac at the back of the heart.
    C. The sinus venosus empties into the right atrium.
    D. The atrium contracts, pumping the blood into the single ventricle.
    E. The ventricle contracts, pumping the blood into the conus arteriosus.
    F. The conus arteriosus branches into right and left truncus arteriosi.
    G. Each truncus arteriosus branches into 3 arteries.
        1. Carotid arch: to head.
        2. Pulmocutaneous artery: to lungs and skin.
        3. Aortic arches (fuse into single dorsal aorta): to the rest of the body.
    H. Oxygenated blood returns from the gills/lungs/skin by the pulmonary veins.
    I. The pulmonary veins empty into the left atrium.
    J. Both atria contract simultaneously.
    K. A three-chambered heart thus mixes the oxygenated and deoxygenated blood.

VII. Frog anatomy.

    A. Mouth.
        1. Large sticky tongue attached to the front of the mouth.
        2. Swallows food (insects) whole.
        3. Nostrils: 2 outside mouth, 2 inside mouth.
        4. Eustachian tubes connect to the ear cavity.
        5. The gullet is at the rear.
        6. The glottis, immediately in front of the gullet, leads to the lungs.
        7. Males have openings in the rear of the lower jaw  to vocal sacs.

    B. Digestive System.
        1. Short esophagus.
        2. Stomach: preliminary digestion.
        3. Pyloric valve controls entrance to the intestine.
        4. Small intestine.
            a. Duodenum.
            b. Ileum.
        5. Short colon (large intestine).
        6. Cloaca.
        7. Anus.
        8. Liver.
            a. Large, 3-lobed organ.
            b. Produces bile, which aids in digestion.
            c. Bile is stored in the gall bladder.
        9. Pancreas.
        10. Mesenteries: transparent membranes which bind the organs to the dorsal body wall.

    C. Nervous System.
        1. Brain contains five lobes common to most vertebrates.
        2. The eye has a variable iris to regulate light input.
        3. A transparent nictitating membrane wipes the eye, keeping it moist.
        4. The tympanic membrane is an eardrum.