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
BIOL 201: Invertebrate Zoology
Chapter 9: Intro to Bilateria
Rob SwatskiAsst. Prof. Biology, HACC-York
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
2
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
Bilateria: Symmetry Planes & Body Axes
4
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
Cephalization
Development of head
Anterior concentration of central nervous system & sensory
structures
Brain is usually 1 or more ganglia
Longitudinal nerve cords: often paired
6
7
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)
8
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
9
Elongation
Shortening
Circular & Longitudinal Musculature10
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
Peristaltic Burrowing
Longitudinal & circular muscles12
Dorsoventral musculature 13
Helical musculature14
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
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
Gut Regions & Specializations
Foregut
Buccalcavity
Pharynx Esophagus
Midgut
Stomach Intestine Cecum
Hindgut
Rectum Cloaca
17
18
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
19
Midgut
Stomach
Enlarged cavity
Extracellular digestion
Intestine
Forms feces
Joins hindgut
Cecum
Outpocketsof stomach or intestine
More SA: intracellular digestion,
absorption, & storage
20
Hindgut
Rectum
Enlargement before anus
Receives indigestible
wastes
Cloaca
A variation of the rectum in some animals
Also receives wastes from
excretory ducts
21
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
22
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
Functions of Coeloms
Houses organs
Hydrostatic skeleton
Circulation & internal transport
Excretion Reproduction
24
25
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
26
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
Parallel Blood Circuit
28
Series Blood Circuit
29
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
30Nephridium animation
Metanephridia
31
Protonephridia
flame cell
32
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.
33
34
35
Spiral Cleavage
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
36
Coelom Formation
Enterocoely
Outfoldings of archenteron pinch off In deuterostomes
Schizocoely
Coelomiccavities form
early in development
In protostomes
37
38
Division of Bilateria
Protostomes
Molluscs, annelids,
& arthropods
Spiral cleavage & schizocoely
Blastoporebecomes
mouth
Deuterostomes
Echinoderms & chordates
Radial cleavage & enterocoely
Blastoporebecomes anus & mouth forms
elsewhere
39
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http://robswatskibiology.wetpaint.com
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