The Phylum Chordata
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This phylum has 3 subphyla
- Urochordata
- Cephalochordata
Craniata
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Chordates are deuterostomes
(sensu strictu)
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Possess four unique characteristics

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

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

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

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

4) Postanal Tail

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

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

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

Subphylum Urochordata (larva)

Subphylum Urochordata (adult)

Subphylum Cephalochordata
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Lancelets
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Laterally compressed

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

Origin of Craniates & Vertebrates
• Fossils – resembling Cephalochordates – Burgess
Shale of British Columbia ~550mya
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Craniates and Vertebrates first appear during the
Cambrian “explosion” ~ 530mya
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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

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

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

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

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

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

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

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

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

Hagfish

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

Lamprey (I)

Lamprey (II)

Lamprey (III)

The First Gnathans
• Classes Placodermi and Acanthodii (extinct)
• Possessed hinged jaws and paired fins
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Jaws developed from skeletal rods supporting some of the more anterior pharyngeal slits
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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

Basic Craniate (from text)

Development of Jaws
(from arch supports 3 & 4)

Jaws and Supports

Acanthodian

Placoderm

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

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

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

Black Tipped Reef Shark

Ray

The “Bony” Fishes
• Formerly in one class (Osteichthyes) but now divided into three classes
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Cloaca absent in all three classes; dioecious; heart is 2-chambered
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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

Coelacanth

Dipnoi (Australian Lungfish)

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

Rainbow Trout

Evolution of lungs & Swim Bladder

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