The Phylum Chordata

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