Practice Questions for Exam 1

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Practice Questions for Exam 1

• Reproduction that occurs when a portion of a
  parent splits off to form a new individual is
• a. Asexual
• b. Budding
• c. Sexual
• d. a and b
• e. None of the above

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• Which one of the following regarding the
  protozoans is false?
• a. They are often colonial
• b. Some have chloroplasts and photosynthesize
• c. Some possess flagella for locomotion
• d. They are metazoan
• e. Some reproduce by fission

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Biology 320Invertebrate ZoologyFall 2005

• Chapter 9 Introduction to Bilateria

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Bilateria

• Majority (99) of eumetazoans exhibit bilateral
  symmetry
• These organisms are lumped into the taxon
  Bilateria
• Has led to the enhancement of neuromuscular
  systems
• Major milestone - cephalization
• Therefore, has allowed the colonization of more
  physiologically challenging habitats
• In particular, terrestrial habitats
• Also allowed the attainment of larger body sizes
• 90m in the blue whale

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Bilateral Symmetry

• Animal has only one plane of bisection
• Known as midsaggital plane
• Produces left and right mirror-image halves
• Body polarized along two axis
• Anterior / posterior (head / tail)
• Ventral / dorsal (belly / back)
• Most likely evolved from organisms who began
  spending time at air / water interface

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• Bilateral symmetry is most often seen in animals
  that move horizontally through habitat
• Useful because food / mates usually randomly
  distributed
• Sessile animals often filter feed
• Radial symmetry is suitable to lifestyle
• Plankton / particulate matter is somewhat
  uniformly distributed

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Cephalization

• Means head development
• Anterior localization of CNS and sensory
  structures
• Brain usually one or more ganglial complexes
• Vertebrates have skull and spinal column to
  protect large CNS
• Exhibited by motile bilateria
• Allows motile animals to detect and pursue food,
  mates, shelter, etc.

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• Mouth at anterior end
• Longitudinal nerve cords
• Usually paired
• Often have giant axons for rapid conductance (low
  resistance), presumably for escape response
• To contrast motile bilateria, sessile bilateria
  usually are not cephalized
• May have radially symmetric portions such as
  crown of tentacles

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Musculature

• As many are motile, often have obliquely / cross
  striated muscle for rapid contraction
• Remember, smooth muscle contracts slowly
• Outer layer of circular muscle almost always
  encloses an inner longitudinal layer
• Especially true in vermiform animals
• Contraction of circular muscles produces
  elongation
• Due to hydrostatic skeleton

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• Longitudinal muscle contractions allow for
• Bending
• Undulating
• Shortening / retraction of posterior end
• Peristalsis
• Coordinated / alternating contractions of
  circular and longitudinal muscles
• Useful for forward propulsion and burrowing
• Other types of musculature
• Dorsoventral flattening
• Helical twisting

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Bilaterian Compartmentalization

• Important for specialization of physiological
  regulation
• Cnidarians first evolved a cavity for
  extracellular digestion
• However, the gastrovascular cavity is not
  specialized
• Carries out multiple functions
• Example, bilaterian gut specializes in digestion
  and absorption
• Bilaterians have evolved other body systems that
  increase physiological compartmentalization
• Coelom
• Hemal system

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The Bilaterian Gut

• More primitive bilaterians still posses a blind
  gut
• Many have a true gut (hollow tube) that is
  composed of specialized regions
• Mouth and foregut ingestion and initial
  digestion
• Midgut chemical digestion and absorption
• Hindgut and anus elimination of wastes,
  reclamation of water, and ion regulation

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• Origins of gut regions
• Foregut and hindgut develop from embryonic
  ectoderm
• Midgut develops from embryonic endoderm becomes
  gastrodermis
• Specialized structures of gut regions
• Foregut
• Buccal cavity chamber that receives food and
  may house teeth
• Pharynx throat that may be protrusible in some
  animals
• Esophagus ciliated portion that links foregut
  and midgut
• Midgut
• Stomach enlarged for digestion
• Intestine forms feces and joins hindgut
• Ceca outpockets of stomach or intestine that
  increase SA for digestion, absorption, and/or
  storage
• Hindgut
• Rectum receives indigestible wastes
• Cloaca name for rectum if gonoducts / excretory
  ducts empty here

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Keep in mind, names of gut regions and structures
are not standardized across the phyla!
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Coeloms

• Fluid-filled cavity lined with mesodermally
  derived epithelium (mesothelium)
• Fluid is known as coelomic fluid
• Circulated by mesothelial cilia or muscular
  contractions
• Functions
• Houses organs
• Hydrostatic skeleton
• Circulation
• Reproduction
• Excretion
• Three types
• Acoelomates typical of small animals as they
  are not diffusion limited
• Pseudocoelomates cavity not entirely lined with
  mesothelium
• Coelomates true body cavity lined with
  mesoderm-derived mesothelium or peritoneum

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• Segmentation occurs when coelom is partitioned
  by
• Mesenteries longitudinal
• Septa transverse
• Benefit is that each segment can be regulated
  individually
• Locomotion
• Loss of some segments may not be life threatening

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Hemal System

• Essentially a circulatory system that consists
  of
• Blood (called hemolymph in most inverts)
• Vessels
• Sinuses
• Blood is usually circulated via muscular
  contractions
• Hemal system is usually absent in small animals
• Some organisms (especially pseudocoelomates) have
  a hemocoel
• Large circulatory sinus that doubles as main body
  cavity

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Excretion

• Larger animals have nephridia (little kidneys)
• Remove nitrogenous waste and osmoregulate
• Wastes are removed w/ water out of a
  nephridiopore
• Variation in nephridia

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Cleavage Patterns

• Diagnostic tool for systematics
• Radial cleavage
• Cleavage planes are parallel or perpendicular to
  axis of zygote
• Occurs in cnidarians, lophophorates, and
  deuterostomes (covered next)
• Spiral cleavage
• Cleavage planes are oblique to axis of zygote
• Occurs in flatworms, molluscs, segmented worms,
  etc.

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Coelom Formation

• Enterocoely
• Outfoldings of archenteron pinch off
• Occurs in deuterostomes
• Schizocoely
• Coelom splits early in development
• Forms segmental coelomic cavities

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Divisions of Bilateria

• Two major divisions
• Protostomia molluscs, annelids, and arthropods
• Deuterostomia echinoderms and chordates
• Protostomes
• Spiral cleavage
• Schizocoely
• Blastopore becomes mouth
• Deuterostomes
• Radial cleavage
• Enterocoely
• Blastopore becomes anus, and mouth forms elsewhere

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