NICHOLLS BIOL 156 - Phylum Chordata - Chordates

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Phylum Chordata - ChordatesInternal skeleton with muscle attachments on outer surfaceAllows for greater range of movement and development of largebody sizeSegmented body planAll chordates have(in embryo or adult)notochorddorsal nerve tubepost-anal tailpharyngeal gill slitsHollow dorsal nerve tube - just below dorsal surfacedifferentiates into brain and spinal cord in vertebratesFlexible dorsal notochord - located just below nerve cordreplaced by vertebral column in vertebrate developmentpersists in some primitive chordates - provides supportPharyngeal gill slits - a clue to the aquatic ancestry of all chordatespharynx connects mouth cavity and esophagus to outsidein terrestrial vertebrate embryos slits often form pharyngeal pouches, become eustachian tubes in some vertebratesA Post-anal tailextends beyond theanuspresent in embryo ifnot in adult formOther animals withtails have a terminalanusPhylum Chordata has three SubphylaSubphylum Urochordata - tunicates or sea squirtsSubphylum Cephalochordata - lanceletsSubphylum Vertebrata - fishes, amphibians, reptiles, birds, mammalsSubphylum Urochordata - Tunicates or “sea-squirts”Most are sessile as adults, may be colonialPossess notochord and nerve cord only in larval stageAdults secrete cellulose “tunic” around bodyAdults are filter feeders lack a body cavity and segmentationCreate water currents with cilia - water drawninto pharynxfood particles trapped inmucus produced by endostyleMany have symbiotic photosynthetic bacteriaMost chordate characteristics are only seen in tunicates in thelarval stageLarvae are free-swimming --after finding a suitablelocation they settle head-firstand transform into the adultformSubphylum Cephalochordata - lanceletsScaleless, fish-like marine organismsNo obvious head, eyes, nose or earsNotochord runs entire length of body and persists in adultsBury in substrate, expose anterior end onlySegmentation of muscles readily visibleHave many more pharyngeal gill slits than fishesFilter-feed throughthe action of cilia inmouth and pharynx -trap food in pharynxSwim with fish-likeundulations of thebodyThe defining chordate characteristics are preserved in the adultlanceletSubphylum Vertebrata - VertebratesAll vertebrates have a distinct head, with skull enclosing a brainName derived from bony vertebral segments in spinal columnVertebral column - protects dorsal nerve cord notochord surrounded and replaced by vertebral columnPossess a cranium - brain caseGroup also called craniate chordatesMost have a bony skeleton, some use only cartilageAll have a closed circulatory systemAll vertebrates also have a unique set of cells that originateabove the neural tube during development - “neural crest cells”Neural crest cells migrate throughout vertebrate body andcontribute to the development of many organsThe vertebralcentra replacethe notochordNeural spinesdevelop tosurround andprotect theneural tubebehind the headBrief overview of vertebrate evolutionFirst vertebrates were marine, lacking jaws or paired fins (550 mya)Jawed fishes then became dominant creatures in the sea (450 mya)Amphibian ancestors first to invade the land (400 mya)Largely replaced by reptiles more suited to live on land (300 mya)Dinosaurs and other reptiles ruled the earth for 150 million yearstheir extinction (65 mya) allowed mammals to become dominantMost livingvertebrate groupshave diversifiedgreatly since theirorigin.The vertebrates are composed of eight classesFour classes are aquatic fishesClass Myxini - HagfishClass Cephalaspidomorphi - LampreysClass Chondrichthyes - Sharks, skates and raysClass Osteichthyes - Bony fishesFour classes are terrestrial tetrapodsClass Amphibia - Salamanders, frogs and toadsClass Reptilia - ReptilesClass Aves - BirdsClass Mammalia - MammalsCharacteristics of FishesGills - extract dissolved oxygen from water around themwater passes over filaments rich in blood vesselsblood moves opposite the flow of water - an efficient systemVertebral columnInternal skeleton with spine surrounding dorsal nerve cordSkeleton may be made of cartilage or boneBrain fully encased in protective skullSingle-loop blood circulationBlood pumped from heart to gillsOxygenated blood from gills passes to rest of bodyHeart is series of fourchambers thatcontract insequenceNutritional deficienciesAll vertebrates are unable to synthesize aromatic amino acidsAll must consume these amino acids in their dietOverview of Fish EvolutionFirst fishes known were jawless with heavy bony armorInternal skeleton was made of cartilageSurvived by lampreys and hagfish (“Agnathans” - no jaws)First jawed fishes occurred 410 million years agoEarly jawed fisheswere replaced bymore agile sharksand bony fishesJaws evolved from modified gill archesthe bony area between gill slitsEvolved as a means to close mouth, and then to capture preyFirst gill arch behind jaw became supporting strutJoined rear of lower jaw to rear of skullAllowed mouth to open very wide, an efficient weaponJawless fishes todayHagfishes are scavengers in coolwaters - especially in deep waterLampreys have two life-styleslarge parasites of other vertebratessmall filter-feeders in freshwater streamsSharks became dominant predators about 300 myaClass Chondrichthyes - sharks, skates, raysSkeleton is made of cartilageHave bony teeth and scales with enamelHave always been large agile predatorsenlarged pectoral fins allow greater lift andmaneuvering during swimmingHave internal fertilizationyoung are well-developed when bornskates and rays have flattened bodies and are more sedentaryClass Osteichthyes - bony fishesEvolved at same time as sharks, but in freshwaterDeveloped heavy skeleton made completely of boneOssification replaces cartilage with boneExternal plates and scales also made of boneAdded swim bladder for respiration/buoyancyPrimitive fishes use swim bladder for respiration - a lungModern fishes use swim bladder for buoyancyModern bony fishes have highly mobile fins, thin scales and symmetrical tailsThey are an extremely successful group and are dominant in freshwater and the seaTwo lineages of bony fishes evolvedRay-finned fishes - most modern fishInternal skeleton of bony rays supportsand stiffens each finNo muscles within finsSwim bladder used for buoyancyLobe-finned fishes - 6 living species5 lungfishes, and the “coelacanth”have fins with muscular lobesswim bladder used for respirationearly form gave rise to amphibiansImportant characteristics of bony fishesLateral line


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NICHOLLS BIOL 156 - Phylum Chordata - Chordates

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