Chondro + Ichthyes = "cartilage fish"
    Devonian to Recent
    ca. 960 spp.

Basic structure
    amphistylic / hyostylic / holostylic jaws
       chondrocranium  -  cartilaginous braincase
            cartilaginous endoskeleton
                adaptation for weight reduction
                note that cartilage precedes bone developmentally but
                              bone preceded cartilage evolutionarily
       polyphyodont dentition
            "multiple tooth generations"
            Tooth whorl is infinitely replaceable
                 specialized placoid scales
                     pulp cavity
                     dentine
                     enamel

pectoral, pelvic, caudal, anal, dorsal fins
    tripartite fin structure
        pterygial elements (basal elements) +
        radial elements +
        ceratotrichia
    pectoral ("shoulder") & pelvic ("hip") girdles divided, fixed
    heterocercal caudal fin - provides thrust
        hypochordal lobe - provides directional control

vertebral centra (sing. centrum)
    neural arches - protect nerves
    hemal arches - protect blood vessels
    "ball & socket" articulaion provides flexibility

    oviparous - egg hatches outside
        / ovoviviparous - egg hatches inside, young "born alive"
           / viviparous - live bearing
           direct development - young born as miniature adults

Circulatory system - two-chambered heart
    sinus venosus  -> atrium + ventricle -> conus arteriosus -> gills

Digestive system
    spiral valve

Diversity of Chondrichthyes

Squaliformes - typical modern sharks (ca. 350 spp.)
    fusiform, pelagic, marine predators
        sometimes benthic or sedentary, rarely freshwater
    5-7 gill openings
    3 pterygial elements
    neutral buoyancy w/o swimbladder
    genital claspers in males (sexual dimorphism)

Variations in feeding style
    euryphagous / stenophagous
    Planktivores - basking shark (Cetorhinus)
    Piscivores
        small scale -  whale shark (Rhinodon)
        large scale - great white shark (Carcharodon)
                [click here for a website on the Great White Shark]
            bite force
            ordered sensory cues
                olfaction
                      directional olfaction in hammerhead shark (Sphyrna)
                vibration
                     lateral line systems
                vision
                  tapetum lucidum works well in reduced light
                    [rods] >> [cones] => light >> colour
                electroreception
                  Ampullae of Lorenzini  detect weak electrical fields
                       fluid fills sacs on rostrum (snout)
          cranial kinesis
                operation of hyostylic jaw
                "JAWS" - human attacks
"Although the species is responsible for an average of 2-3 non-fatal attacks on swimmers, surfers, and divers each year, its role as a menace is exaggerated; more people are killed in the U.S. each year by dogs than have been killed by white sharks in the last 100 years." (Dr. Douglas Long, California Academy of Sciences)

Rajiformes - skates & rays (> 400 spp.)
    dorso-ventrally flattened, benthic marine crustacivores
    crushing-plate dentition
    gill slits ventral
    pectoral fins expanded as "wings"
        other fins much reduced
    electric organ in rays (Raja)
        defence (200V discharge)
        species recognition?
            species-specific frequency (Hz) or voltage (V)

Chimaeriformes - Ratfish  (ca. 30 spp.)
    odd, scaleless, benthic & shallow water invertebrate feeders
    4 gill slits w/ operculum
    holostylic jaw
        upper jaw fused to cranium
    cephalic claspers in males