Chordates

CHORDATES

ConceptsChordates: Lancelets are nonvertebrate chordates w/ 4 chordate characteristics as adults: 1. notochord2. a dorsal tubular nerve cord3. pharyngeal pouches4. postanal tailVertebrates:-Notochord is replaced by vertebral column-have head region-An endoskeleton-paired appendages

Fishes3 groups:1. jawless (hag fishes and lampreys)2. cartilaginous (sharks and rays)3. bony (ray-finned and lobed-finned) Most modern-day fishes are ray-finned e.g. trout and perch

Amphibians (e.g. frogs and salamanders) evolved from lobed-finned fishes.Modern-day reptiles( e.g. turtles, lizards, and snakes) are the remnants of an ancient group that evolved from amphibians.The shelled egg reptiles, which contains extraembryonic membranes, is an adaptation for reproduction on land.

There is a close evolutionary relationship between birds and reptiles. However, birds maintain a constant body temperature.Birds have feathers and skeletal adaptations that enable them to fly.Mammals, which evolved from reptiles, were present when dinosaurs existed. They did not diversify until the dinosaurs became extinct.

Mammals are vertebrates with hair and mammary glands. Hair helps them maintain a constant body temperature.Mammals are classified according to methods of reproduction:Monotremes lay eggsMarsupials have a pouch into which the newborn crawls and develops furtherPlacental mammals retain offspring in a uterus until birth

Phylum ChordataConsists of 45,000 species of animalsRepresent less than 5% of the total number of species alive todaye.g. familiar animals:Sharks, goldfish, toads, turtles, sparrows, antelopes and humans

The 4 basic characteristics of a chordate;NotochordNerve cordPharyngeal pouchpostanaltail

1. Notochord-a dorsal supporting rod- Located below the nerve chordThe majority of vertebrates have an embryonic notochord that is replaced by the vertebral column during development

2. A dorsal tubular nerve cord -the anterior portion becomes the brain in most chordates. In vertebrates, the nerve cord, often called the spinal cord, is protected by vertebrae.

3. Pharyngeal pouchesThese are only seen during embryonic development in most vertebratesIn nonvertebrate chordates, fishes, and amphibian larvae, the pharyngeal pouches become functioning gills (respiratory organs of aquatic vertebrates) [Water --- passing into the mouth & pharynx--- gill slits(supported by arches)Terrestrial vertebrates-pouches are modified for various purposesIn humans-the 1st pair of pouches become auditory tubes. The 2nd pair become tonsils. The 3rd/4th pairs become the thymus and parathyroid glands.

4. Postanal tailA tail in the embryo if not in the adult extends beyond anus

Nonvertebrate Chordates:The notochord persists in the adult form and is never replaced by the vertebral column2 subphyla of lower chordates1. subphylum Cephalochordata-[25 species]Lancelets2. subphylum Urochordata- [2,000 species]sea squirtse.g. lancelets [Genus Branchiostoma]Amphioxus-former genus now used as common nameThe nonvertebrate chordates include-1.the lancelets -found in shallow water along most coast, sandy,muddy substrates w/ anterior mouth and gill apparatus exposed, have segmented muscles [feed:microscopic particles---mouth---gill slits---atrium that opens at the atripore

CephalochordataLancelet

sea squirts- ocean floor. tunicates-tunic that look like thick-walled, squat sacs [ The larva is bilaterally symmetrical & has 4 chordate charateristics].Metamorphosis produces the sessile adult w/ incurrent and excurrent siphon

UrochordateTunicate

VertebratesFeatures of vertebratesLiving endoskeleton with vertebral columnClosed circulatory systemPaired appendagesEfficient respiration and excretionHigh degree of cephalization[vertebrates are adapted for an active lifestyle

OsteichthyansRay-Finned Fishes and Lobe-Fins

LobeFins

FishesCharacteristics:Equatic ectothermsSkin covered with scalesFins for swimmingSingle-loop cardiovascular pathwayBreathe with gillsLay eggs

AmphibiansFeatures:Usually tetrapodsUsually lungs in adultMetamorphosisSmooth and moist skinThree-chambered heartEctothermyMost lay eggs in water

ReptilesFeatures:Usually tetrapodsLungs with expandable rib cageLeathery-shelled amniotic eggDry-scaly skinendothermy

BirdsFeatures:FeathersHard-shelled amniotic eggFour-chambered heartUsually wings for flyingAir sacsendothermy

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Monotremes

Marsupials

Eutherians (Placental Mammals)Eutherians are commonly called placental mammals becausetheir placentas are more complex than those of marsupials.Eutherians have a longer pregnancy than marsupials. Youngeutherians complete their embryonic development within theuterus, joined to their mother by the placenta. The eutherianplacenta provides an intimate and long-lasting association bern'eenthe mother and her developing young.

MammalsFeatures:Body hairDifferentiated teethWell-developed brainUsually live births and newborn dependencyMammary glandsEndothermyInternal development

NotochordChordates are named for a skeletal structure, the notochord,present in all chordate embryos as well as in some adult chordates.The notochord is a longitudinal, flexible rod locatedbehveen the digestive tube and the nerve cord. It is composedof large, fluid-filled cells encased in fairly stiff, fibrous tissue.TIle notochord provides skeletal support throughout most ofthe length of a chordate, and in larvae or adults that retain it,it also provides a firm but flexible structure against which muscles can work during swimming. In most vertebrates, amore complex, jointed skeleton develops around the ancestralnotochord, and the adult retains only remnants of the embryonicnotochord. In humans, the notochord is reduced to gelatinousdisks sandwiched between the vertebrae.The nerve cord of a chordate embryo develops from a plate ofectoderm that rolls into a tube located dorsal to the notochord.The resulting dorsal, hollow nerve cord is unique tochordates. Other animal phyla have solid nerve cords, and inmost cases they are ventrally located. The nerve cord of achordate embryo develops into the central nervous system:the brain and spinal cord.The digestive tube ofchordates extends from the mouth to theanus. The region just posterior to the mouth is the pharynx.In all chordate embryos, a series of pouches separated bygrooves forms along the sides of the pharynx. In most chordates,these grooves (known as pharyngeal clefts) developinto slits that open to the outside of the body. Thesepharyngeal slits allow water entering the mouth to exit thebody without passing through the entire digestive tract. Pharyngeal slits function as suspension-feeding devices in manyinvertebrate chordates. In vertebrates (with the exception ofvertebrates with limbs, the tetrapods), these slits and thestructures that support them have been modified for gas exchangeand are known as gill slits. In tetrapods, the pharyngealclefts do not develop into slits. Instead, they play animportant role in the development of parts of the ear andother structures in the head and neck.Chordates have a tail that extends posterior to the anus, althoughin many species it is greatly reduced during embryonicdevelopment. In contrast, most nonchordates have adigestive tract that extends nearly the whole length of thebody. The chordate tail contains skeletal elements and muscles,and it helps propel many aquatic species in the water.

*Urochordata, class Ascidiacea- Distaplia occidentalis, Diplosoma*The least derived craniatelineage that still survivesis Myxini, the hagfishes(Figure 34,9). Hagfisheshave a skull made of cartilage,but they lack jaws andvertebrae. They swim in asnakelike fashion by usingtheir segmental muscles toexert force against their notochord,which they retain in adulthood as a strong, flexiblerod of cartilage. Hagfishes have a small brain, eyes, ears, and anasal opening that connects with the pharynx. Their mouthscontain tooth-like formations made of the protein keratin.All of the 30 living species of hagfishes are marine. Measuringup to 60 em in length, most are bottom-dwelling scavengersthat feed on worms and sick or dead fish. Rows of slime glandson a hagfish's flanks secrete a substance that absorbs water,forming a slime that may repulse other scavengers when a hagfishis feeding (see Figure 34.9). When attacked by a predator, ahagfish can produce several liters ofslime in less than a minute.The slime coats the gills of the attacking fish, sending it into retreator even suffocating it. Several teams of biologists and engineersareinvestigating the propertiesofhagfish slime in hopesof producing an artificial slime that could act as a space-fillinggel. Such a gel might be used, for instance, to curtail bleedingduring surgery.*Lampreys (Petromyzontida)represent the oldest livinglineage of vertebrates. Likehagfishes, lampreys may offer clues to early chordateevolution but have also acquiredunique characters.Thereareabout35speciesof lampreys inhabiting various marine and freshwater environments (Figure 34.10). Mostare parasites that feed by clamping their rowld, jawless mouth ontothe flank of a live fish. They then use their rasping tongue topenetrate the skin ofthe fish and ingest the fish's blood..As larvae, lampreys live in freshwater streams. The larva isa suspension feeder that resembles a lancelet and spendsmuch of its time partially buried in sediment. Some species oflampreys feed only as larvae; following several years instreams, they mature sexually, reproduce, and die within a fewdays. Most lampreys, however, migrate to the sea or lakes asthey mature into adults. Sea lampreys (Petromyzon marinus)have invaded the Great Lakes over the past 170 years and havedevastated a number of fisheries there.The skeleton of lampreys is made of cartilage. Unlike thecartilage found in most vertebrates, lamprey cartilage containsno collagen. Instead, it is a stiff protein matrix. The notochordof lampreys persists as the main axial skeleton in the adult, asit does in hagfishes. However, lampreys also have a cartilaginouspipe around their rodlike notochord. Along the length ofthis pipe, pairs ofcartilaginous projections related to vertebraeextend dorsally, partially enclosing the nerve cord.

*a side-to-side bending of the body, a trait inherited from theearly terrestrial tetrapods (Figure 34.21a). Paedomorphosis iscommon among aquatic salamanders; the axolotl, for instance,retains larval features even when it is sexually mature (seeFigure 25.20).Anurans, numbering about 5,420 spe