Biology 202 - Lecture 4

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Biology 202 - Lecture 4
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2011-03-21 21:49:14
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Lecture - 4
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  1. Infraphylum: Hyperotreti, Class: Myxini
    • 20 extant species of deep-sea scavengers (hagfishes)
    • have most of the basic characteristics of vertebrates ...except vertebrae!
    • in addition, hagfishes have several 'derived' (specialized) traits:
    • - mucous glands and thread cells
    • - degenerate eyes
    • - "teeth" on tongue
    • - unique feeding
  2. Infraphylum VertebrataClass: Petromyzontida
    • includes >40 species of parasitic fishes (lampreys)
    • demonstrate the basic vertebrate design as larvae:
  3. But as adults, they have many specializations, e.g.:
    • digestive system specialized for parasitism
    • tidal ventilationSeveral species are pests of commercial and sport fisheries.
  4. The Origin of Jaws
    Most vertebrates have jaws and paired appendanges: when did these evolve?
  5. Gnathostomes
    • probably evolved in the mid Ordovician (~480 mya)
    • underwent a second duplication of the Hox genes;
    • possess numerous derived characteristics (specializations) compared to agnathans:
  6. Skeletal System
    • jaws, derived from the first two pharyngeal arches- teeth
    • two pairs of appendages (pectoral, pelvic), probably derived from skin folds
  7. Respiratory System
    - gills with counter-currentexchange
  8. Reproductive System
    gametes excreted via thearchinephric duct (sperm)and oviduct (eggs)
  9. Nervous System
    • myelinated axons
    • lateral line (canal containing groups of hair cells) for sensing motion in water
    • may also detect electrical fields
  10. Do any living species exhibit just the basic gnathostome design?
    • No. Gnathostomes diversified during the Ordovician (~450mya), and include two derived lineages:
    • Class Chondrichthyes("cartilageous fishes")
    • "bony fishes" and tetrapods.
  11. Class: Chondrichthyes
    What’s special about the Chondrichthyes?
    • Living chondrichthyans have several distinctive characteristics:
    • placoid scales
    • teeth replacedcontinuously
    • skeleton calcified, but notossified; fins supported by keratinized rods
    • ventral mouth; intestine with spiral valve; liver often oil-filled for buoyancy
    • osmoregulation via urea
    • eggs generally develop in a protein sac(oviparous), but some species are ovoviviparous[female retain eggs during development] or viviparous [developing young are supported by a placenta]; internal fertilization via pelvic claspers
    • well-developed electroreception; reflective layer in retina enables vision in dim light; strong olfactory sensitivity
  12. Is there much variation within Chondrichthyes?
    Chondrichthyes underwent three major radiations; two major groups remain, including ~900 species:
  13. (1) Holocephali
    • chimeras, ratfishes
    • feed on crustaceans
    • upper jaw fused to cranium; teeth modified into grinding plates; one gill slit
  14. (2) Elasmobranchii
    • modern sharks, rays, skates
    • active predators and planktivores
    • jaw-opening mechanismenables jaw protrusion andsuction; stiff fins and heterocercal tail provide lift; multiple gill slits

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