CHAPTER 23
Chordates
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The Chordates: Characteristics
Structural Plan
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Name chordata comes from the notochord
5 Hallmark chordate characteristics
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Dorsal, tubular nerve cord (spinal cord)
Notochord (in some replaced by vertebrae)
Pharyngeal slits (gills)
Endostyle (aka - thyroid gland)
Postanal tail (coccyx)
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Characteristics of Chordates
Muscle segments
Tail
Anus
Notochord
Hollow nerve cord
Mouth
Pharyngeal pouches
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23-4
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The Chordates: Characteristics
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Chordates share features with some
invertebrates:
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Bilateral symmetry
Coelom
Metamerism
Cephalization
Deuterostomes (like echinoderms)
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23-6
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Traditional and Cladistic Classification of the Chordates
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Traditional classification
Protochordata (no skull) are separated from
Vertebrata that have a skull
 Vertebrates may be divided into Agnatha
(jawless) and Gnathostomata (having jaws)
 Gnathostomata is subdivided into Pisces
with fins and Tetrapoda, usually with two
pair of limbs
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Five Chordate Hallmarks
Notochord
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Always found at some embryonic stage
First part of the endoskeleton to appear in
the embryo
Serves as an axis for muscle attachment
In protochordates (no skull) and jawless
vertebrates,
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Persists throughout life
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23-9
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Five Chordate Hallmarks
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In vertebrates
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Series of cartilaginous or bony vertebrae
form notochord
In most vertebrates
Notochord displaced by vertebrae
 Remnants may persist between or within
vertebrae
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Five Chordate Hallmarks
Dorsal Tubular Nerve Cord
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Anterior end enlarges to form the brain
Cord is produced in embryo by
infolding of ectodermal cells on the
dorsal side of body
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Five Chordate Hallmarks
Pharyngeal Pouches and Slits
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Pharyngeal slits lead from pharyngeal cavity
to the outside
Form by the inpocketing of the ectoderm and
the evagination of endoderm of pharynx
In aquatic chordates
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2 pockets break through to form pharyngeal slit
In amniotes
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Pockets may not break through and only grooves
are formed
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Phyla Chordata Cladogram
23-13
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Five Chordate Hallmarks
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In tetrapods
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Pharyngeal pouches give rise to a variety of
structures, including the Eustachian tube, middle
ear cavity, tonsils and parathyroid glands
Perforated pharynx functions as filterfeeding apparatus in protochordates
Fishes added a capillary network with thin
gas-permeable walls
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Led to evolution of gills
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Five Chordate Hallmarks
Endostyle or Thyroid Gland
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Endostyle or its derivative, the thyroid
gland, found in all chordates
Some cells in endostyle secrete
hormones similar to the thyroid gland
of adult lampreys and the remainder of
vertebrates
23-15
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Five Chordate Hallmarks
Postanal Tail
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Postanal tail, plus musculature, provided
motility
Efficiency increased in fishes but became
smaller or vestigial in later lineages
23-16
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Subphylum Urochordata: Tunicata
Diversity of Urochordata
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Approximately 3000 species of tunicates identified
Occur in all seas and at all depth
In most species, only the larval form bears all the
chordate hallmarks
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During adult metamorphosis
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Adults lose many of these characters
Notochord and tail disappear
Dorsal nerve cord is reduced
Urochordata include
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All Tunicates including Sea Squirts
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Adult
Tunicate
Colony
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Nonvertebrate Chordates
Tunicate
Siphon to mouth
Pharynx with gill slits
Siphon from anus
Anus
Tunic
Intestine
Reproductive organs
Heart
Stomach
Adult
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Sea Squirt:
Larval Stage has ALL 5 Chordate characteristics
23-20
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Tunicate:
Illustrating Endostyle
and Pharyngeal slits
(characteristics remaining
In adult tunicate)
23-21
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Sea Squirt - Adult
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Subphylum Cephalochordata
Diversity
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Lancelets
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Live in sandy bottoms of coastal waters
around the world
Also known as amphioxous
About 25 species of amphioxus are described
5 occur in North American coastal waters
Many zoologists consider amphioxus a living
descendant of ancestors that gave rise to both
cephalochordates and vertebrates
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Slender, laterally flattened, translucent animals
about 5–7 cm long
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Amphioxus:
Shows ALL 5
chordate
characteristics
Post-anal
Tail
23-24
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2 Amphioxus
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Subphylum Vertebrate (Craniata)
Adaptations That Guided Vertebrate Evolution
 Earliest vertebrates
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Were substantially larger
Considerably more active
Characterized by increased speed and mobility
Higher activity level and size of vertebrates
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Requires structures specialized in the location,
capture, and digestion of food and adaptations
designed to support a high metabolic rate
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Subphylum Vertebrate (Craniata)
Musculoskeletal Modifications
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Most vertebrates possess skeleton of
cartilage or bone
Endoskeleton permits almost unlimited body
size
Endoskeleton allows attachment of
segmented muscles
Endoskeleton probably composed initially of
cartilage and later gave way to bone
23-27
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Spinal
cord
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Vertebral Column
And Head skeleton
Common in vertebrates
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Subphylum Vertebrate (Craniata)
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Endoskeleton of living hagfishes,
lampreys, sharks and their kin, and
even some “bony” fishes, such as
sturgeons, mostly composed of
cartilage
Structural strength of bone is superior
to cartilage
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Makes it ideal for muscle attachment in
areas of high mechanical stress
Perhaps bone evolved, in part, as a
means of mineral regulation
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Subphylum Vertebrate (Craniata)
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Some of the most primitive fishes, including
Ostracoderms and placoderms were partly
covered in a bony, dermal armor
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Modified in later fishes as scales
Most vertebrates are protected with
keratinized structures derived from the
epidermis
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Reptilian scales, hair, feathers, claws, and horns
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Subphylum Vertebrate (Craniata)
Physiology
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Modifications of digestive, respiratory, circulatory,
and excretory systems to meet increased metabolic
demand
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To manage increased ingestion of food
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Gut shifted from movement of food by ciliary
action to muscular action
Accessory digestive glands, the liver and
pancreas, produced secretions that aided
digestion
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Subphylum Vertebrate (Craniata)
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Transport of nutrients gases, and other
substances was enhanced by
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Ventral 3-chambered heart
 Sinus venosus
 Atrium
 Ventricle
Included hemoglobin
Vertebrates possess paired kidneys to
remove metabolic waste products and
regulated body fluid composition
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Subphylum Vertebrate (Craniata)
New Head, Brain, and Sensory Systems
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Shift from filter feeding to active predation
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Anterior end of nerve cord enlarged as a
tripartite brain
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Required new sensory, motor, and integrative
controls for location and capture of larger prey
Forebrain, midbrain, and hindbrain
Brain was protected by cartilaginous or bony
cranium (skull)
Paired special sense organs for vision,
equilibrium, and sound evolved
Other receptors that evolved
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Mechanoreceptors, chemoreceptors,
electroreceptors, and olfactory receptors
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23-34
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Subphylum Vertebrate (Craniata)
Neural Crest and Ectodermal Placodes
 Development of vertebrate head and special sense
organs lead to rise of these innovations
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Neural crest
 Derived from ectodermal cells in the embryonic neural tube
 Contributes to formation of: cranium, pharyngeal skeleton, tooth
dentine, some cranial nerves, ganglia, Schwann cells, and some
endocrine glands
 May also regulate development tooth enamel and pharyngeal
muscles
Ectodermal placodes
 Plate-like ectodermal thickenings on either side of neural tube
 Give rise to olfactory epithelium, lens of eye, inner ear epithelium,
some ganglia, some cranial nerves, lateral-line
mechanoreceptors, and electroreceptors
 Placodes also induce formation of taste buds
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Evolutionary History
The Earliest Vertebrates
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Ostracoderms
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Until recently, earliest known vertebrate fossils
were armored jawless fishes called ostracoderms
Found in the late Cambrian deposits in United
States, Bolivia and Australia
Small, heavily armored, jawless, and lacked
paired fins
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Early Jawless Fish: Ostracoderms
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Evolutionary History
Vertebrates
 Gnathostomes
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All living and extinct jawed vertebrates
Agnathans, defined by the absence of jaws
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Living agnathans (jawless vertebrates), the
lampreys and hagfishes
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Shark Structure
Evidence that gill supports resemble jaw structure:
Evolutionists believe that the jaws arose from the cartilage gill supports
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Early Jawed Fish: Placoderm and Acanthodian
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Osteichthyes
JAWS
Jaws helped vertebrates
to become successful
predators.
VERTEBRAE
Vertebrates have a
segmented backbone.
Amphibia
Mammalia
Aves
mammals
bony fish
Chondrichthyes
cartilaginous fish
lamprey
Agnatha
FOUR LIMBS
Four limbs let animals
move from the water to
life on land.
FEATHERS
Feathers insulate birds
from the cold and allow
for flight.
HAIR
Hair helps mammals
to maintain constant
body temperatures by
roviding insulation
from the cold.