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Fundamentals of Biology

Lesson 21

Chordates, Vertebrates, and Fish


I.  Phyla, Subphyla.

    A. Hemichordata.
    B. Chordata.
        1. Urochordata.
        2. Cephalochordata.
        3. Vertebrata.

II. Hemichordata - acorn worms.

    A. Invertebrates; formerly classed as chordates.
        1. Possess pharyngeal gill slits (compare with pharyngeal pouches in chordates).
        2. Once thought to have a notochord.

    B. General Characteristics.
        1. All are marine.
        2. Some are burrowing worms
        3. Others are sessile with tentacles.
        4. Three part body.
            a. Proboscis - ciliated; used for burrowing and feeding.
            b. Collar - mouth.
            c. Trunk - digestive tract.
        5. Both ventral and dorsal nerve cords.

III. Chordata.

    A. General Characteristics.
        1. Dorsal notochord at some stage of life.
            a. Tough, flexible, rod-like structure running the length of the body.
            b. Serves as primary support.
        2. Dorsal tubular nerve chord; brain at anterior end.
        3. Pharyngeal pouches at some stage in life.
            a. Folds of skin along the neck region.
            b. Aquatic organisms: they open into gill slits.
            c. Others: do not open.
        4. Ventral heart.
        5. Endoskeleton.
        6. Postanal tail at some stage in life.

    B. Urochordata - tunicates.
        1. All are marine.
        2. Only larva has notochord.
        3. Ascidians are sessile, others are planktonic.
        4. Ascidians (sea squirts).
            a. Secrete cellulose covering.
            b. Filter feeders.
            c. Many colonial.

    C. Cephalochordata - amphioxus (lancelets).
        1. Marine.
        2. Look like fish with too many gill slits.
        3. Bury all but head in sand.
        4. Cilia pull food into mouth.

IV. Vertebrata.

    A. General Characteristics.
        1. Vertebral column: more or less replaces the notochord.
        2. Liver, endocrine organs, kidneys.
        3. Prominent cephalization.

    B. Terms.
        1. Oviparous: eggs hatch outside the body.
        2. Viviparous: offspring are nurtured in the mother’s body by a placenta.
        3. Ovoviviparous: eggs hatch inside the mother’s body, but there is no placenta.
        4. Ectothermic (cold-blooded): cannot generate their own body heat.
        5. Endothermic (warm-blooded): generate their own body heat.

    C.  Classes of Fish.
        1. Agnatha (Jawless Fish).
        2. Chondrichthyes (Cartilaginous Fish).
        3. Osteichthyes (Bony Fish).

V.  Agnatha.

    A. General Characteristics.
        1. No jaws.
        2. Long, narrow, cylindrical body.
        3. No paired fins.
        4. Cartilage skeleton.

    B. Lampreys.
        1. Temperate aquatic habitats.
        2. Two dorsal fins and a tail fin.
        3. Parasitic.
            a. Sucking mouth, rasping tongue covered with teeth.
            b. Mouth holds it to the host.
            c. The tongue breaks the skin.
            d. It then sucks the blood and body fluids of the host.
            e. Usually does not kill its prey.
        4. Larval form lives as long as seven years as an amphioxus-like filter feeder.
        5. Adult lives only a few years; dies after spawning upriver.

    C. Hagfish.
        1. Temperate and semitropical Atlantic and Pacific habitats.
        2. Eat dead, diseased, or disabled fish as well as invertebrates.
        3. Can bite using cartilage rods on either side of the mouth.
        4. No larval stage.

VI.  Shared Characteristics of Chondrichthyes and Osteichthyes.

    A. Jaws.
    B. Two pairs of lateral fins.
        1. Pectoral fins (front).
        2. Pelvic fins (rear).
        3. Permit a greater degree of balance and maneuverability.
    C. Unpaired dorsal, anal and tail fins.

VII.  Chondrichthyes.

    A. Sharks.
        1. Covered with tiny scales.
        2. Carnivorous.
            a. Some are filter feeders.
            b. These feed on plankton.
        3. Digestive System.
            a. Teeth are enlarged, specialized scales.
            b. Swallow food whole – as much as a dolphin or part of a horse.
            c. Digestion mainly in the stomach.
            d. Absorption in the intestine.
            e. Intestine is short and fat, but surface are increased by a spiral valve.
        4. Enormous liver.
            a. Stores lipids.
            b. Lipid content regulates buoyancy.
        5. Respiratory System.
            a. Lack pumping muscles to draw a water current across the gills.
            b. Thus they need to swim constantly or they will drown.
        6. Nervous System.
            a. Large olfactory organs in their heads to detect chemicals.
            b. Lateral line system.
                i. Pressure receptors on the side of the body.
                ii. Detect vibrations or sound waves.
            c. Ampullae of Lorenzini.
                i. Specialized lateral line receptors in the head.
                ii. Long mucus-filled canals.
                iii. Detect electric fields.
        7. Reproductive System.
            a. Internal fertilization.
            b. Many ovoviviparous, but some are viviparous and others are oviparous.
            c. Females produce a few large eggs with a lot of yolk.
            d. Males use claspers, extensions of the pelvic fins, to pass sperm to the females.
            e. Eggs of oviparous sharks are covered with leathery coat.
            f. In most species the eggs remain in the oviduct until the baby shark is ready to be born.
            g. The sand tiger embryos eat the other embryos and eggs coming from the ovary.

    B. Rays and Skates.
        1. Pectoral fins are enlarged into wings that propel them through the water.
        2. Stingrays have sharp, poisonous spines that cause painful wounds.
        3. Electric rays stun prey or enemies with an electric shock.
        4. Manta rays can get as large as 20 feet across.

VIII.  Osteichthyes.

    A. Covering.
        1. Most have scales.
        2. Mucus covers the scales.
            a. Waterproofs.
            b. Protects from parasites.
            c. Lubricates for swimming.
        3. Chromatophores: branched cells that produce pigments.
            a. Rearranging the pigment in the cell changes the color of the fish.
            b. Color changes in response to temperature, diet, excitement, physical condition.
            c. Countershading.
                i. Dorsal side is dark to blend in with the dark deep water.
                ii. Ventral side is light to blend in with the bright suface.

    B. Movement.
        1. Moves mainly by whipping motion of its body.
        2. Many have an air bladder, a thin-walled sac containing gases to regulate buoyancy.

    C. Digestive System: mouth, pharynx, esophagus, stomach, intestine, anus.

    D. Respiratory System.
        1. Operculum: a plate behind the eye on each side of the head.
        2. Gills are underneath the operculum.
            a. Two rows of thin gill filaments on a cartilage band called a gill arch.
            b. Each filament is a stack of thin plates.
            c. These plates are covered by a thin epithelium and filled with blood vessels.
        3. Two-stage process of respiration.
            a. Operculum closes, gill arches expand drawing water into the mouth.
            b. Mouth closes, gill arches contract, operculum opens forcing water over the gills
        4. O2 and CO2 are exchanged, up to 85% efficient.

    E. Circulatory System.
        1. Two-chambered heart.
        2. Sinus venous empties into the atrium.
        3. Atrium pumps blood into the ventricle.
        4. Ventricle pumps blood through the ventral aorta.
        5. Blood passes through the gills; gives up CO2 and takes in O2.
        6. Oxygenated blood enters the dorsal aorta.
        7. Blood passes through the body tissues and returns to the sinus venous.

    F. Nervous System.
        1. Brain with major lobes; spinal cord.
        2. Smell.
            a. One of sharpest senses.
            b. Olfactory sacs behind the nose.
        3. Lateral line system.
            a. Pressure receptors on the side of the body.
            b. Detect vibrations or sound waves.
        4. Taste and touch on the tongue.
        5. Sight.

    F. Reproductive System.
        1. Most are oviparous, but some are ovoviviparous and some are viviparous.
        2. Female
            a. Paired ovaries.
            b. Can produce 100,000's of eggs.
        3. Female spawns.
        4. Male covers the eggs with milt – milky fluid containing sperm.
        5. Few hours to 90 days for eggs to hatch.

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