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The Phylum Chordata

The Phylum Chordata. This phylum has 3 subphyla - Urochordata - Cephalochordata Craniata Chordates are deuterostomes ( sensu strictu ) Possess four unique characteristics. 1) Dorsal Hollow Nerve Cord. Forms from a dorsal ectoderm tube Forms the CNS (brain & spinal cord)

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The Phylum Chordata

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  1. The Phylum Chordata • This phylum has 3 subphyla - Urochordata - Cephalochordata • Craniata • Chordates are deuterostomes (sensustrictu) • Possess four unique characteristics

  2. 1) Dorsal Hollow Nerve Cord • Forms from a dorsal ectoderm tube • Forms the CNS (brain & spinal cord) • Other animal phyla have ventral solid nerve cord(s)

  3. 2) Notochord • Longitudinal, flexible cartilagenous rod • Located between nerve cord and gut

  4. 2) Notochord • Extends most of the length of the organism • A simple ENDOSKELETON • For some chordates this is all they have (thus ‘invertebrate’ chordates) • VERTEBRATE chordates develop a more complex jointed skeleton • BUT vertebrates still have vestiges of the notochord (intervertebral discs !)

  5. 3) Pharyngeal Slits • Early chordates – these connected to the digestive tract; used for FILTER FEEDING • Later became modified for gas exchange • Present (vestigial) in our early embryo stage

  6. 4) Postanal Tail

  7. 4) Postanal Tail • Digestive tract extends most of the body length in the majority of non-chordates • Tail extends beyond the anus in chordates • Contains skeletal elements • Contains muscles • Major propulsive force in many aquatic chordates • Other uses for the tail ?????

  8. Subphylum Urochordata • Invertebrate chordates • Called Tunicates (some called sea squirts) • Tunic is made of a cellulose-like carbohydrate called tunicin • Filter Feeders • Free swimming as larvae • Sessile as adults (adhere by ‘head’ region to objects)

  9. Subphylum Urochordata • Highly modified as adults • Scarcely resemble other chordates • During metamorphosis they lose the tail, notochord & nerve cord (‘brain’ atrophies) • Retain pharyngeal slits • Closed circulatory system • Possess heart & blood cells (no hemoglobin) • Most primitive of the chordates (?????)

  10. Subphylum Urochordata (larva)

  11. Subphylum Urochordata (adult)

  12. Subphylum Cephalochordata • Lancelets • Laterally compressed

  13. Subphylum Cephalochordata • Adults somewhat resemble urochordate larvae • All 4 chordate traits persist • Filter feeders; tentacles around mouth • Marine; burrow tail first • Feeble swimmers; fish-like movements (sinusoidal) • Serially arranged muscle segments

  14. Origin of Craniates & Vertebrates • Fossils – resembling Cephalochordates – Burgess Shale of British Columbia ~550mya • Craniates and Vertebrates first appear during the Cambrian “explosion” ~ 530mya • Early Craniates and Vertebrates possessed all four chordate characteristics and were filter feeders • Probably derived from a Urochordate-like ancestor similar to a tunicate larva • Paedogenesis resulted in a larva achieving sexual maturity and it did not undergo metamorphosis

  15. Paedogenesis • Precocious attainment of sexual maturity in a morphologically juvenile organism • If successful, natural selection would have reinforced the absence of metamorphosis • These larvae were active and natural selection would favor the most active • Actively foraging organisms benefit from good sense organs • Cephalization is a benefit

  16. Craniate / Vertebrate Characteristics • All possess the four basic chordate traits (at least at some stage) • Cephalization with a highly specialized brain • The brain is covered by or enclosed by a skull • MOST Craniates possess a vertebral column that encloses the nerve cord • MOST possess jaws • Skeleton is an endoskeleton

  17. The Endoskeleton • Composed of cartilage, bone or both • Grows with the organism • Cells secrete and rearrange the matrix • AXIAL SKELETON - skull plus vertebral column (plus ribs & breastbone, if present) • APPENDICULAR SKELETON may be present (supports fins or limbs)

  18. Additional Traits (I) • Closed circulatory system • Heart with 2, 3 or 4 chambers • Arteries, capillaries, veins • Red blood cells (with hemoglobin) for O2 transport • Blood oxygenated through skin or more commonly via gills or lungs

  19. Additional Traits (II) • Possess kidneys (compact excretory structures) • Reproduction usually sexual • Dioecious (= unisexual) • Gender control variable (XY, WZ, environmental) • Some can change gender • Parthenogenesis found in most classes (but not common)

  20. Subphylum Craniata • Nine or ten extant classes • Mixini • Petromyzontidae • Chrondrichthyes • Antinopterygii, Actinistia, Dipnoi • Amphibia • Reptilia • Aves (?) • Mammalia

  21. The Agnathan Classes (I) • Oldest fossilized craniates were agnathans • Mud-suckers/filter feeders • Living forms lack paired appendages and external armor • No covers for gill slits • Two-chambered heart • Class Mixini (hagfishes) • Class Petromyzontida (lampreys)

  22. The Agnathan Classes (II) • Hagfishes (30 species) are eel shaped • Lack rasping mouthparts • Some feed on sick or dead fish or on worms • Marine • Skull of cartilage • No vertebrae (are “craniate invertebrates”)

  23. Hagfish

  24. The Agnathan Classes (III) • Lampreys (about 35 species) are eel shaped • Young are usually suspension (filter) feeders • Young live in freshwater • Adults usually marine but some stay in freshwater • Adults have rasping mouth parts • Attach to live fish and are parasitic • Possess a cartilaginous tube around the notochord with pairs of projections (Vertebrae); thus a “craniate vertebrate”)

  25. Lamprey (I)

  26. Lamprey (II)

  27. Lamprey (III)

  28. The First Gnathans • Classes Placodermi and Acanthodii (extinct) • Possessed hinged jaws and paired fins • Jaws developed from skeletal rods supporting some of the more anterior pharyngeal slits • Jaws allow for more varied feeding habits • Vertebrate jaws work UP and DOWN (arthropod jaws work Side to Side) • Remaining slits functioned as major gas exchange sites

  29. Basic Craniate (from text)

  30. Development of Jaws(from arch supports 3 & 4)

  31. Jaws and Supports

  32. Acanthodian

  33. Placoderm

  34. Class Chondrichthyes (I) • The cartilaginous fishes (about 750 species) • Lack of bone in skeleton is considered a derived condition • Sharks, rays, skates, and more • No swim bladder • Must swim to stay up in the water column • Some have added buoyancy due to large amounts of oil stored in the liver

  35. Class Chondrichthyes (II) • Marine animals – evolved in the sea • Paired fins, well-developed jaws; most are carnivores (a few filter feeders) • No opercula (external gill covers) • Scaled; Teeth evolved from modified scales • Reproduce sexually; fertilization internal; dioecious • Oviparous, Ovoviviparous or Viviparous • Claspers on male’s pectoral fins transfer sperm into the female’s reproductive tract

  36. Class Chondrichthyes (III) • Cloaca present • Heart two-chambered • Short digestive tract; spiral valve present • Good vision but no color vision • Sharks have a fusiform body • Rays and skates are dorsoventrally flattened

  37. Black Tipped Reef Shark

  38. Ray

  39. The “Bony” Fishes • Formerly in one class (Osteichthyes) but now divided into three classes • Cloaca absent in all three classes; dioecious; heart is 2-chambered • Class Actinistia – coelacanths (believed to have been extinct for 65 million years) – Two (?) extant species; a lobe-fin group that evolved in freshwater and then moved to the ocean • Class Dipnoi – lungfishes – another lobe-fin group; 3 genera and 7 species; Freshwater; use lungs for respiration; Gave rise to tetrapods

  40. Coelacanth

  41. Dipnoi (Australian Lungfish)

  42. Class Antinopterygii • Largest group of vertebrates (~30,000 spp) • Most of our familiar fish species • Body covered by flattened bony scales • Evolved in freshwater; many moved to the ocean; some have returned to freshwater • A few species spend a portion of their lives in freshwater and in saltwater • Possess a swim bladder for buoyancy – it developed from lungs

  43. Rainbow Trout

  44. Evolution of lungs & Swim Bladder

  45. Gas Exchange in Fishes • Chondrichthyes – swimming important – forces water into mouth and out over gills • Bony Fish – water drawn into mouth and forced out over gills by movement of the OPERCULUM and contraction of muscles within the gill chambers

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