Intro' to Bilateria

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Intro. to Bilateria

• Chapter 9

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The Issue of Body Symmetry
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Bilateral Symmetry mirror images found only in
a mid-sagittal vertical plane
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In animals having radial symmetry, body polarity
parallels the environmental gradient.
Bilateral animals move perpendicular to the
environmental gradient and thus have two
polarities.
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Two Polarized Axes in Animals Having Bilateral
Symmetry

• Anterior-posterior body axis is a result of
• Polarization along the locomotor gradient (front
  vs. back)

• Dorsal-ventral body axis is a result of
• Polarization along the environmental gradient
  (water vs. substrate)

Neither of these two axes thus result in mirror
images.
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Changes in Body Geometry
Polarization along locomotor gradient
Mirror images
Polarization along the environmental gradient
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As a Result of the Two Polarities

• Anterior end is cephalized with a concentration
  of cerebral ganglia and sense organs.
• The dorsal side is modified for protection and
  camouflage
• The ventral side is modified for locomotion
  (cilia or muscles) or adhesion (as in many
  molluscs)

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What are the advantages to an animal in moving
across the environmental gradient?
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Movement Across the Environmental Gradient May
Have

• increased the likelihood of finding food because
  food is generally found in patches
• increased the likelihood of encountering mates

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Do we see any radially symmetrical animals moving
across the environmental gradient?

• A hint of things to come?
• Portuguese Man-of War
• By-the-wind Sailor
• Ctenophores or comb jellies

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Bilateral Symmetry

• Strongly correlated with organisms that move over
  the top of the substrate
• And in sessile, suspension-feeding animals that
  are dependant on a unidirectional current
• as in the next slide

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Animals Having Bilateral Symmetry
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Some Bilateral Animals Have Reverted Back to
Radial Symmetry

• Those that have secondarily taken up a sessile
  existence have developed a secondary radial
  symmetry ex. - many echinoderms, especially the
  sessile sea-lilies and in annelids known as
  feather-dusters, etc.
• Those worms that burrow below the substrate. They
  escape the environmental gradient above ground
  for uniformity below.

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These Suspension Feeders Have Evolved Secondary
Radial Symmetry Because Food is Coming From All
Directions
Feather Duster
Sea Lily
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How Are Resources Targeted?

• Cephalization
• Anterior brain
• Paired, anterior sensory neurons for
  triangulation of prey
• Longitudinal nerve cords
• Paired motor neurons to stimulate musculature
• Giant axons in larger inverts (ex. squid)

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How Are Resources Pursued?
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Contractions of the Body-Wall Musculature Against
the Hydrostatic Skeleton
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The Mechanics of Burrowing
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Peristaltic Burrowing
Longitudinal muscle contracting at anchors
Circular muscle contracting at narrow areas
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The Bilaterian Gut

• Concepts of regionalization and specialization
• From embryonic ectoderm foregut and hindgut
• From embroynic endoderm midgut

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Gut, Proboscis and Introvert
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What is a coelom?

• a fluid-filled cavity lined with mesoderm
  (peritoneum) that sits between the body wall and
  the gut wall
• provides a new, third internal compartment that
  permits physiological specialization
• May be separated into smaller cavities (in each
  segment) that is separated from others by a
  septum
• Contains coelomic fluid

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How is Coelomic Fluid Different from Blood?

• Coelomic Fluid
• water, ions, solutes
• germ cells that become gametes (in some)
• phagocytic cells
• intracellular respiratory proteins (pigments)

• Blood
• Same
• no germ cells
• phagocytic cells
• respiratory proteins may be intracellular as in
  hemoglobin or may be extracellular

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Internal Fluid-Transport Systems

• Coelomic Systems
• when large, the hemal system is reduced
• embryonic origin epitheliaum of mesoderm
• contains coelomic fluid
• exs. Platy., Nematodes, Annel.

• Hemal (Blood) Systems
• when large, the coelomic system is reduced
• embryonic origin blastcoel (between ectoderm
  and mesoderm)
• contains blood or hemolymph
• exs. Arthro., Moll., Echino., Chordates

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Hemocoel vs. Coelomic Cavity
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Convection vs. Diffusion Methods of Internal
Transport

• Convection-based System
• May Involve a Coelom or a Hemal System
• Used by larger bilteria

• Diffusion-based System
• Neither a coelom or hemal system is present
• Used by smaller bilateria with plenty of surface
  area per volume

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Internal Transport Convection vs. Diffusion
Large BilateriaHemal Transport System
Large Bilateria Coelom TransportSystem
Small BilateriaDiffusion only
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Bilaterian Hemal Systems

• Parallel Circuit
• Major vessels form an elongate loop with dorsal
  vessel above and ventral vessel below
• Parallel vertical vessels located between dorsal
  and ventral vessels
• Ex. Annelid worms

• Series Circuit
• Body cavity converted to hemocoel (no major
  vesselsother than dorsal heart)
• Blood moves through body organs in series fashion
  providing nutrients to each as it passes through
• Exs. Arthropods and Molluscs

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Parallel vs. Series Blood Circuits
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Excretory Systems

• Protonephridial
• Typically in smaller bilateria lacking a coelom,
  blood vessels or both
• Filtration thus occurs at the proximal end of the
  protonephridium
• Selective reabsorption and secretion occur
  farther down the protonephridium

• Metanephridial
• Typically in larger bilateria
• Filtrate (Initial Urine) produced by filtration
  from blood through filter due to blood pressure
• Blood enters metanephridium and selective
  reabsorption and secretion occurs ? (Final Urine)

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Excretion Protonephridial System
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Excretion Metanephridial System
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End