Phylum Chordata

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  • Created by: rosieevie
  • Created on: 06-11-17 13:09

Phylum Chordata

Includes both vertebrates and non-vertebrate chordates

3 sub-phyla:

  • Urcochordata (tunicates)
  • Cephalochordata (lanclets)
  • Vertebrata (inc. lamprey and hagfish)
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Chordate Origins

Origin in a tunicate-like (urochordata) ancestral form - Delsuc et al. (2006)

  • Cephalochordates and vertebrates considered most related - morphological similarities and increased complexity
  • Grouping of tunicates and vertebrates into clade Olgactores
    • Structure of cadherin genes
    • Limited by relatively few characters considered
    • Other studies limited by no. chordate species studied
  • Delsuc - extended data to 13 chordates
  • Provided evidence that urochordates closest living relatives
  • Phylogenetic analyses
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Chordate Radiation

Greater variety of habitats than any other phylm, established dominance in some

EX - bony fish

Groups have invaded land then returned to sea e.g. cetaceans

Complicated mechanisms/behaviour:

  • Resisting dessication
  • Skeletal support (endoskeletons) - no limiting growth
  • Some endothermic - optimise muscular activity
  • Developed nervous systems - quick adaptation to environmental changes

= Well-established and top of food chains

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Chordate Embryology

Deutrostomes (ecinoderms are only invertebrate deutrostomes) - first blastopore opening becomes anus

Principle embryo characteristics:

  • Stiff notochord
  • Single dorsal hollow nerve cord (component of nerve system)
  • Pharyngeal slits (gill slits/throat structures)
  • Triploblastic layers (ectoderm, mesoderm and endoderm)

Somites - blocks of muscle which attach to notochord/spinal chord

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Notochord

Always found at some embryonic stage of all chordates

First part of endoskeleton to develop

  • Give bilateral symmetry
  • Axis for muscle attachmment - anchorage
  • Some degree of flexibility

May persist throughout life or lost in metamorphosis

NOT A PRECURSOR OF VERTEBRAE (derived from mesoderm cells lateral to notochord)

PERSISTS AS INTERVERTEBRAL DISCS (displaced by vertebrae)

Structure - concentric rings of 3 layers:

  • Elastic outer covering
  • Fibrous notochordal sheath - oreintation changes = resists forces from all directions = flexibility
  • Vaculoated cells in center = hydrostatic core = elasticity
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Hollow Dorsal Nerve Cord

Single cord

Dorsal of alimentary canal (digestive tract)

Ventral in invertebrates

Anterior end enlarges to form brain

Neurulation:

  • Notochord induces overlying neuroectoderm to invaginate on dorsal side 
  • Neural tube formed
  • Nerve cord produced
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Pharyngeal Pouches/Slits

Pharyngeal slits lead from pharyngeal cavity to outside of the body

Formation:

  • Inpocketing of ectoderm
  • Outpocketing of pharynx endoderm
  • Join in aquatic chordates = slit for filter feeding and gills
  • 3 grooves persist in amniote chodates = Eustachian tubes, middle ear cavity, tonisls, parathyroid glands
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Endostyle

Endostyle - organ assisting lower-chordates (uro, cephalo, lamprey larvae) in filter feeding

  • Longitudinal ciliated fold in vental pharyngeal wall
  • Secretes mucus - accumulate food particles
  • Passes particles to digestive tract

Evolved to form and secrete proteins which bind iodine

  • Evolutionary origin of the vertebrate endocrine gland
  • Tyrosine tightly bound to iodine - form called MIT in chordates
  • Acts as a monomer for condensation polymerisation in active thyroid hormone (T3/T4)
  • Associated with region 7 of endostyle
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Post-Anal Tail

Continues after the anus

Has notochord and nerve cord running into it

Works with myomeres and notochord = mobility and stability

Propulsion in lower chordates but vestigial in later lineages

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Urochordates

Tunicates - sea squirts, salps and larvaceans

Exoskelton tunic

Filter feed with perforated pharyngeal basket

Adults metamorphosis - tadpole-like larvae exhibit key chordate features (notochord, dorsal hollow nerve cord and post-anal tail)

Larvae swim from hatching site and settle - planktotrophic

Larvacea group are free-living - retain chordate features

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Cephalochordata

1 extant Genus Branchiostoma (lancelets)

  • Well defined head
  • Swim with fish-like locomotion using straited myomeres
  • Myomeres arranged longitudinally around notochord
  • Mostly partially buried
  • Filter feed through pharyngeal basket

Demonstrate intermediate features between invertebrates and vertebrates:

  • Body wall - epidermic of columnar epithelium and connective tissue (associated w/ muscus layer in vertebrates - protection)
  • Myomeres - chevron shaped blocks, reduces the coelom
  • Notochord - major role in locomotion as anchorage for muscles
  • Caugal fin - propulsive fin, also a storage organ for nutritional reserves for gametes
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Branchiostoma Circulatory System

No heart - use 4 vesselled circulation powered by smooth muscle

Peristaltic sequential contraction

Closed system and flow similar to fish - partial system as blood is linked to 2 major organs (intestinal arteries and caecal - head - area)

No blood pigment cells

Functions mainly as nutrient distribution

Diffusion may occur across gills but most from wall of metapleural folds (thin flaps of body wall anterior to atriopore)

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Branchiostoma Excretory System

Numerous protonephridia - accumulate nitrogenous waste

  • Grouped in large numbers
  • Analagous to start of the kidney
  • Structural similaries to invertebrate solenocytes - probably convergent evolution

Removed via nephridioduct through pore in atrium

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Branchiostoma Nervous System

Dorsal nerve cord - extends most of body

Anterior slight expansion, cerebral vrsicle at base of oral hood

Segmentally arranged nerves arise from cord along body - typical vertebrate pattern of dorsal and ventral roots - serves myomeres to control blood movement

Simple senses - tactile, chemoreceptors and ocelli (eye spots)

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Branchiostoma Reproduction

Dioecious

Gonads ventral to pharynx

Gametes exit via atriopore

External fertilisation

Deutrostome development

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Origin of Vertebrates

Centrum functionally replaces notochord

Notochord becomes central part of intervertebral discs (nucleus pulposus)

Tissues surrounding it ossifies - patches which become precursors to vertebrae

Vertebral arch - circle of bone around canal through which spinal cord passes

Centrum - solid part of vertebra, to which arches and processes are attached

  • Vertebral arches - arisen in stem group giving rise to agnathans and gnathostomes
  • Centra appear later - only gnathostomes
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Craniata External Characteristics

Cephalisation - drives distinct bilateral symmetry and nerve system advancements

  • Responsible for success - skull protects adaptable brain

Paired limbs - originate as swimming stabilisers but now evolved to jointed limbs (terrestrial fine movement)

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Craniata Internal Characteristics - Living Endoske

Endoskeleton allows continuous growth - avoids problems and size limitation associated with moulting

Jointed scaffolding for muscles (levers) = greater muscle mass = stronger and cushioning from impact

Skull and ribcage protect vital organs 

May also have external keratin/bony plates

Cartilage likely first endoskeletal material - faster growth than bone

Bones store minerals (P, Ca) and mechanical strength for terrestrial life

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Craniata Internal Characteristics - Circulation

Highly efficient closed circulatory systems

Multi-chambered hearts

Blood cells with haemoglobin 

= Greater efficiency of gaseous exchange, digestion and excretion 

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Craniata Internal Characteristics - Physiology

Complex and intergrated organ systems

Compelx respiratory systems developed from a perforated pharynx in filter feeding developing to a muscular pharynx for water (gills) or air (lungs)

Complete digestive systems w/ accessory glands for enzymes and hormones for rapid processing of large food volume, storage and detoxification

Short digestion times in predators to move quickly - food heavy and impairs movement

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Craniata Internal Characteristics - Embryos

Close association betwen renal and reproductive systems - derived from the same tissues

Extraembryonic membrane enclosing the yolk sac produced in 2 ways:

  • Anamniotic eggs (fish/ambphibia)
    • Protective membranes of embryo produced by reproductive tract of female 
    • No extensive outer membrane
    • Egg not needed to be protected against dessication
  • Amniotic eggs (birds, reptiles and mammals)
    • 3 additional extraembryonic/postembryonic membranes
      • Amnion - surrounds entire contents of egg
      • Chorion - protects egg from dessication
      • Allantois - provides water to egg
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Craniata Internal Characteristics - Nervous System

Advanced nervous system w/ well-defined brains

Increased sensory, motor and intergrative controls associated with changes in genetics (Hox genes)

Anterior end of nerve cord = tripartite brain (forebrain, midbrain, hindbrain)

Rise of cerebellum - increase in complex behaviour and social interactions 

Paired and complex sensory organs

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Hox Genes

Regulates the expression of a hierachy of genes - control the process of long-axis body development fron front to back

Duplication of entire hox complex in vertebrates = dominance of brain and evolution of morphological complexity

4 Hox gene clusters in gnathastomes 

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Vertebrate Evolutionary Pathways

Garstang's Hypothesis of Chordate Larval Evolution

  • Paedomorphosis - reproductive in juvenile form
  • Larvae failed to metamorphosis into adult form - developed gonads
  • Neoteny - retaining some juvenile features in adult form
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