biol 201 chp 9 introduction to bilateria
DESCRIPTION
This is a lecture presentation for my BIOL 201 Invertebrate Zoology students on Chapter 9: Introduction to Bilateria (Invertebrate Zoology, 7th Ed. by Ruppert, Fox, & Barnes, 2004). Rob Swatski, Assistant Professor of Biology, Harrisburg Area Community College - York Campus, York, PA. Email: [email protected] visit my website, BioGeekiWiki, for more biology learning resources: http://robswatskibiology.wetpaint.comVisit my Flickr photostream for anatomy model photographs! http://www.flickr.com/photos/rswatski/Thanks for looking!TRANSCRIPT
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BIOL 201: Invertebrate Zoology
Chapter 9: Intro to Bilateria
Rob SwatskiAsst. Prof. Biology, HACC-York
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Bilateria
Most (99%) eumetazoans display bilateral symmetry
(lumped into Bilateria taxon)
Led to enhancement of neuromuscular
systems: cephalization!
Allowed colonization of more
physiologically challenging habitats
– LAND!
Also allowed growth of larger body sizes
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Bilateral Symmetry
Has only 1 plane of bisection: midsagittal
plane
Produces left & right mirror-image halves
Body polarized along 2 axes: anterior-
posterior axis & dorsal-ventral axis
Most likely evolved from organisms that
spent more time at air-water interface 3
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Bilateria: Symmetry Planes & Body Axes
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Functional Origin of Bilateral
SymmetryCommon in animals
that move horizontally through
habitat
Useful because food & mates are often
randomly distributed
Radial symmetry is more common in
sessile filter-feeders
Radial symmetry works well: plankton
& particulates are more uniformly
distributed 5
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Cephalization
Development of head
Anterior concentration of central nervous system & sensory
structures
Brain is usually 1 or more ganglia
Longitudinal nerve cords: often paired
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Cephalization
Allows motile animals to detect & pursue food,
mates, shelter, etc.
Longitudinal nerve cords: often paired
Giant axons: rapid conductance & low resistance – escape
response
In contrast, most sessilebilateria are not
cephalized & usually have radially symmetric
structures (crown of tentacles w/cilia)
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Musculature
Often have obliquely / cross-striated muscle for rapid contraction
Outer circular muscle usually encloses an inner longitudinal
muscle layer (esp. in worms)
Contraction of circular muscles produces
elongation
Due to hydrostatic skeleton
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Elongation
Shortening
Circular & Longitudinal Musculature10
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Musculature, cont.
Longitudinal muscle contractions allow for: bending, undulating, shortening, retraction
Peristalsis: coordinated / alternating contractions of circular & longitudinal
muscles
Used for forward propulsion & burrowing
Other types of musculature:
dorsoventral (flattening) & helical (twisting)11
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Peristaltic Burrowing
Longitudinal & circular muscles12
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Dorsoventral musculature 13
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Helical musculature14
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Compart-mentalization
Important for more specialized physiological
regulation
Cnidarians 1st evolved a cavity for extracellular digestion – but their
GVC is not specialized
Bilaterian gut isspecialized for digestion
& absorption
Other systems evolved that increase physiological
compartmentalization: coeloms & hemal
systems 15
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BilaterianGut
Many have a true gut (open-ended hollow tube) consisting of specialized regions
Mouth & foregut: ingestion of food,
enzyme secretion, & physical digestion
Midgut: chemical digestion (hydrolysis)
& absorption
Hindgut & anus: formation, storage, & elimination of wastes; water reclamation &
ion regulation 16
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Gut Regions & Specializations
Foregut
Buccalcavity
Pharynx Esophagus
Midgut
Stomach Intestine Cecum
Hindgut
Rectum Cloaca
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Foregut
Buccal cavity
Receives food
May have teeth
Pharynx
Muscular tube
May be protrusible;
used in feeding, digging
Esophagus
Links foregut &
midgut
Muscular & may be
ciliated
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Midgut
Stomach
Enlarged cavity
Extracellular digestion
Intestine
Forms feces
Joins hindgut
Cecum
Outpocketsof stomach or intestine
More SA: intracellular digestion,
absorption, & storage
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Hindgut
Rectum
Enlargement before anus
Receives indigestible
wastes
Cloaca
A variation of the rectum in some animals
Also receives wastes from
excretory ducts
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4 Types of BilaterianCirculatory Systems
Gastrovascular Cavity
Gut lined with gastrodermis; gut
tube & ceca
Hemal
Connective tissue compartment lined with basal lamina;
tubular vessels and/or hemocoelic
sinuses
Coelomic
True body cavity lined with mesothelium; small or large cavities, vessel-
like canals
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Coelom
Fluid-filled cavity or canal: unique to Bilateria
Lined with epithelium derived from mesoderm
(mesothelium)
Filled with coelomicfluid
Fluid circulated by mesothelial cilia or
muscular contractions of body-wall 23
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Functions of Coeloms
Houses organs
Hydrostatic skeleton
Circulation & internal transport
Excretion Reproduction
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Segmentation
Segments: bilateral pairs of fluid-filled
coelomic cavities along length of body
Compartments separated by mesenteries
(longitudinal) & septa(transverse)
Benefit: individual regulation of segments
for specialized functions
Plays role in locomotion
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HemalSystem
A simple circulatory system consisting of: blood (hemolymph),
vessels, & sinuses
Blood circulated via circular musculature
and/or 1 or more hearts
Hemal systems are usually absent in
small animals (acoelomates)
Hemocoel: large sinus acting as the main body cavity
(pseudocoelomates)27
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Parallel Blood Circuit
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Series Blood Circuit
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Excretion
Larger animals have ciliated tubules called
nephridia (little kidneys)
Remove nitrogenous wastes (urine) &
osmoregulate
Wastes are flushed out with water through a
nephridiopore
Different types of nephridia: filtration, secretion, & storage
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Metanephridia
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Protonephridia
flame cell
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Cleavage Patterns
Radial Cleavage
Cleavage planes are parallel or
perpendicular to polar axis of zygote
In cnidarians, lophophorates, &
deuterostomes
Spiral Cleavage
Cleavage planes are oblique to polar axis of
zygote
In flatworms, molluscs,
segmented worms, etc.
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Spiral Cleavage
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Gastrulation(Triploblastic)
Ectoderm
Epidermis Covers body, secretes cuticle &
exoskeleton, contains sensory,
nervous, & glandular cells
Mesoderm
MesotheliumLines coelom,
forms muscles,
mesenteries, gonads, & CT
Endoderm
Gastro-dermis
Lines midgut, contains ciliated,
secretory, absorptive, & storage
cells
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Coelom Formation
Enterocoely
Outfoldings of archenteron pinch off In deuterostomes
Schizocoely
Coelomiccavities form
early in development
In protostomes
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Division of Bilateria
Protostomes
Molluscs, annelids,
& arthropods
Spiral cleavage & schizocoely
Blastoporebecomes
mouth
Deuterostomes
Echinoderms & chordates
Radial cleavage & enterocoely
Blastoporebecomes anus & mouth forms
elsewhere
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Creditsby Rob Swatski, 2010
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