Introduction to Chordates

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Introduction to Chordates

• Chapter 15

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Basics

• Bilateral symmetry
• Segmented body
• Three germ layers
• Well-developed Coelom
• Endoskeleton
• Complete digestive system
• Closed circulatory system

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Chordate Characters

1. Notochord
2. Pharyngeal pouches
3. Postanal tail
4. Nerve cord (dorsal)

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Notochord

• Flexible rod-like structure
• First part of endoskeleton (in embryo)
• Axis for muscle attachment
• In most verts, its replaced by vertebrae

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Pharyngeal pouches

• Pharyngeal slits vary in degree and function
• Aquatic chordates leads to outside of cavity
• Amniotes forms only groves
• Fish became gills

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Postanal tail

• Tail and musculature provide motility
• Fish highly evolved
• Other groups become vestigial

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Nerve cord

• Dorsal to alimentary canal
• Tubular
• Brain on anterior end
• Formed from ectoderm

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SP Urochordata Tunicates
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SP UrochordataTunicates

• Found at all ocean depths
• Largely sessile
• Larval forms have all chordate features
• Adults loose many of these
• Notochord and tail disappear
• Dorsal nerve cord is reduced
• Tunic (test) protective covering around animal

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Adult Tunicates
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Adult and Tadpole Larva of Urochordate
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SP UrochordataExample

• Ascidians sea squirts
• More common intertidal animals
• Can be found attached to rocks
• Colonial and solitary forms
• Feeding
• Filter feed with mucus net

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SP CephalochordataLancelets

• Marine
• Found in sediments near costal waters
• Feeding
• Filter feed with mucus net

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LanceletAmphioxus
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SP CephalochordataBauplan

• Long slender, translucent
• Has four chordate characters
• Is though to be living descendant of ancestor
  that gave rise to vertebrates

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SP Vertebrata

• AKA Craniata all groups have a cranium
• braincase
• Thought to be monopyletic
• Share other important characteristics

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SP Vertebrata

• Fossil invertebrate chordates are rare and known
  primarily from two fossil beds.
• Pikaia from Cambrian Burgess Shale

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Haidouella 530 MYO China
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Haidouella 530 MYO China
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The Early Vertebrates

• Ostracoderms- Armed Jawless Fishes 505 MYO

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SP Vertebrata

• Living Endoskeleton
• Efficient Respiration (Pharynx)
• Advanced Nervous System
• Paired limbs

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Living Endoskeleton

• Grows with body
• No molting
• Efficient use of materials
• Muscle attachment
• Protect body and body protects it
• Made from
• Bone
• Cartilage

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Efficient Respiration (Pharynx)

• Pharynx is result of shift from filter feeding
• Increased metabolic rate
• Addition of capillaries
• Muscular aortic arches

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Advanced Nervous System

• Predation favored more complex nervous system
• Eyes with lenses
• Paired eyes
• Two innovative adaptations
• Neural crest-forms cranium
• Specialized Sense Organs- nose, eyes, ears.

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Paired limbs

• Pectoral and pelvic appendages
• Swimming stabilizers
• Jointed limbs
• Work well in terrestrial environments

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The Big Picture

• Four major evolutionary events binds the
  chordates into a group
• Many of these allowed chordates to be successful
  on land
• Urochordates as adults appear to be sponges or
  cnidarins, but as larvae they have all structures
  shared by chordate
• Remember what it means to be a vertebrate