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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
Infraphylum VertebrataClass: Petromyzontida
- includes >40 species of parasitic fishes (lampreys)
- demonstrate the basic vertebrate design as larvae:
But as adults, they have many specializations, e.g.:
- digestive system specialized for parasitism
- tidal ventilationSeveral species are pests of commercial and sport fisheries.
The Origin of Jaws
Most vertebrates have jaws and paired appendanges: when did these evolve?
- probably evolved in the mid Ordovician (~480 mya)
- underwent a second duplication of the Hox genes;
- possess numerous derived characteristics (specializations) compared to agnathans:
- jaws, derived from the first two pharyngeal arches- teeth
- two pairs of appendages (pectoral, pelvic), probably derived from skin folds
- gills with counter-currentexchange
gametes excreted via thearchinephric duct (sperm)and oviduct (eggs)
- myelinated axons
- lateral line (canal containing groups of hair cells) for sensing motion in water
- may also detect electrical fields
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.
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
Is there much variation within Chondrichthyes?
Chondrichthyes underwent three major radiations; two major groups remain, including ~900 species:
- chimeras, ratfishes
- feed on crustaceans
- upper jaw fused to cranium; teeth modified into grinding plates; one gill slit
- modern sharks, rays, skates
- active predators and planktivores
- jaw-opening mechanismenables jaw protrusion andsuction; stiff fins and heterocercal tail provide lift; multiple gill slits