Download Keep in mind, names of gut regions and structures are not

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

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
Biology 320
Invertebrate Zoology
Fall 2005
Chapter 9 – Introduction to
Bilateria
Bilateria

Majority (99%) of eumetazoans
exhibit bilateral symmetry

These organisms are lumped into
the taxon Bilateria

Has led to the enhancement of
neuromuscular systems

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Therefore, has allowed the
colonization of more physiologically
challenging habitats
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Major milestone - cephalization
In particular, terrestrial habitats
Also allowed the attainment of larger
body sizes
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90m in the blue whale
Bilateral Symmetry
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Animal has only one plane of
bisection
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Body polarized along two axis
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Known as midsaggital plane
Produces left and right mirrorimage halves
Anterior / posterior (head / tail)
Ventral / dorsal (belly / back)
Most likely evolved from
organisms who began spending
time at air / water interface

Bilateral symmetry is
most often seen in
animals that move
horizontally through
habitat

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Useful because food /
mates usually randomly
distributed
Sessile animals often
filter feed
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Radial symmetry is
suitable to lifestyle
Plankton / particulate
matter is somewhat
uniformly distributed
Cephalization

Means “head development”
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Anterior localization of CNS
and sensory structures
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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
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Longitudinal nerve cords
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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
Musculature

As many are motile, often
have obliquely / cross striated
muscle for rapid contraction


Outer layer of circular muscle
almost always encloses an
inner longitudinal layer


Remember, smooth muscle
contracts slowly
Especially true in vermiform
animals
Contraction of circular muscles
produces elongation

Due to hydrostatic skeleton

Longitudinal muscle
contractions allow for:

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Peristalsis

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
Bending
Undulating
Shortening / retraction of
posterior end
Coordinated / alternating
contractions of circular and
longitudinal muscles
Useful for forward
propulsion and burrowing
Other types of musculature


Dorsoventral – flattening
Helical – twisting
Bilaterian Compartmentalization

Important for specialization of physiological regulation

Cnidarians first evolved a cavity for extracellular
digestion

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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
The Bilaterian Gut


More primitive bilaterians still
posses a blind gut
Many have a true gut (hollow
tube) that is composed of
specialized regions

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Mouth and foregut – ingestion
and initial digestion
Midgut – chemical digestion
and absorption
Hindgut and anus –
elimination of wastes,
reclamation of water, and ion
regulation

Origins of gut regions

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Foregut and hindgut develop from embryonic ectoderm
Midgut develops from embryonic endoderm; becomes gastrodermis
Specialized structures of gut regions

Foregut
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Midgut
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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
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
Keep in mind, names of gut regions and structures
are not standardized across the phyla!
Coeloms

Fluid-filled cavity lined with mesodermally derived epithelium
(mesothelium)
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Functions
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Fluid is known as coelomic fluid
Circulated by mesothelial cilia or muscular contractions
Houses organs
Hydrostatic skeleton
Circulation
Reproduction
Excretion
Three types

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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

Segmentation occurs
when coelom is
partitioned by:


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Mesenteries – longitudinal
Septa – transverse
Benefit is that each
segment can be
regulated individually


Locomotion
Loss of some segments
may not be life
threatening
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
Excretion




Larger animals have
nephridia (little kidneys)
Remove nitrogenous
waste and osmoregulate
Wastes are removed w/
water out of a
nephridiopore
Variation in nephridia
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.
Coelom Formation

Enterocoely

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Outfoldings of
archenteron pinch off
Occurs in
deuterostomes
Schizocoely

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Coelom splits early in
development
Forms segmental
coelomic cavities
Divisions of Bilateria

Two major divisions

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Protostomes
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Protostomia – molluscs, annelids, and arthropods
Deuterostomia – echinoderms and chordates
Spiral cleavage
Schizocoely
Blastopore becomes mouth
Deuterostomes



Radial cleavage
Enterocoely
Blastopore becomes anus, and mouth forms elsewhere