The Triploblasitc, Acoelomate
Body Plan
Zoology Chapter 10
Acoelmates, Pseudocoelomates,
and Coelomates
All animals in this chapter are:
1.
2.
3.
Triploblastic – have three primary
germ layers
Acoelomate – without a coelom
Classified into three phyla:
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Phylum Platyhelminthes (flatworms)
Phylum Nemertea (unsegmented)
Phylum Gastrotricha (bottom dwellers)
Phylum Platyhelminthes:
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Flatworms
Most common =
planarian
Contains over 34,000
species
Currently, no uniquely
defining characters
(synapomorphies) in this
phylum
Adult size from 1 mm or less
to 25 m
Live in marine, freshwater,
and damp terrestrial habitats
Flatworm Characteristics:
Bilateral Symmetry
 Cephalization
 Most cells are close to
external environment
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materials can pass easily
into and out of their bodies
(via diffusion)
Rely on diffusion for
respiration, excretion, and
circulation
Three Classes of
Platyhelminthes:

Free-living (not parasites)
flatworms:
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Class Turbellaria
Parasitic Species:
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Class Trematoda
Class Cestoidea
Class Turbellaria
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Mostly free-living
bottom dwellers in
freshwater and
marine
environments
Crawl on stones,
sand, or vegetation
Named for the
turbulence that
their beating cilia
create in the water
Class Turbellaria cont.
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Over 3,000 species
Few terrestrial species live in humid
tropics and subtropics
Less than 1 cm long (rare
terrestrial/tropic = up to 60 cm long)
First group of bilaterally symmetrical
animals to evolve
Feeding
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Carnivores (small
invert.),
scavengers (dead), some
herbivores (algae)
Sensory cells on their
heads help detect food far
away
Digestive cavity has a
single opening through
which food and waste
pass
Feeding cont.
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Pharynx – muscular
tube pumps food from
the mouth into the
digestive cavity or gut
Highly branched gut
transports food to all
parts of the body

(this is lacking in
many parasitic
species)
Locomotion
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Cilia on epidermal cells help to glide through
the water and over the bottom of a stream or
pond
A layer of mucus is laid down to aid in
adhesion and help cilia gain traction
Locomotion cont.
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Muscle cells are
controlled by the
nervous system
Allows animal to
move & react rapidly
to environmental
stimuli
Dorsoventral muscles
maintain flatness
(allows for diffusion)
Exchanges with Environment:

Do not have respiratory organs ∴
done via diffusion
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Respiratory gases (CO2 and O2)
Metabolic wastes (ammonia)
Depends on environment
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Marine = osmotic equilibrium
Freshwater = hypertonic
Excretion:
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Protonephridia is a network of fine tubules
for excretion of metabolic wastes.
Flame cells are ciliated and induce
currents to push fluids through tubules.
Nephridiopore
An opening on the outside of the body from
which tubules eventually merge
Nervous System & Sense Organs:
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Subepidermal nerve plexus (resembles
cnidarians but depends on species)
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Mechanoreceptor: (excited by pressure) at
anterior end detects body position(due to
gravity)
Cerebral ganglia: more centralized nerve
net (“brain”)
Longitudinal nerve cords – ladder-like
appearance (evolutionary advancement
to a nervous system)
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Auricles – sensory
lobes on side of
head aid in food
location
(chemoreceptor)
Ocelli – eye spots;
orient in direction
of light
(photoreceptor)
See how it works…
Asexual Reproduction
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Budding or fission
Regeneration
Sexual Reproduction
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Monoecious: both testes and
ovaries (hermaphroditic)
Eggs develop inside the body and
are shed in capsules called cocoons
Weeks later, the eggs hatch and
grow into adults.
Practice “penis fencing” to
determine which flatworm carries
the fertilized eggs
See it here http://www.youtube.com/watch?v=5fx-YgcP8Gg
Turbellaria
review:
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What type of
symmetry?
How do they
feed?
What type of
sensory organs?
How do they
reproduce?
Parasitic
worms:
Class Trematoda
 Class Cestoidea
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Class Trematoda
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8,000 parasitic
species (aka –
flukes)
Internal parasites
(endoparasitic)
Complex life cycles
specialized in
parasitism in animal
or human tissues
One or more suckers
around anterior end
(oral sucker)
The species that infect humans:
Schistosomes & non-Schistosomes
Dicrocoelium dendriticum and Opisthorchis sp. liver flukes of
mammals
blood flukes, Schistosoma spp. are among most widespread and
serious parasites of humans
Schistosomiasomes (blood flukes)
Class Cestoidea
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aka – tapeworms
Most highly specialized
class
of flatworms
Intestinal parasites
No digestive tract ∴
reside in digestive
system of vertebrates
Absorb nutrients across
body wall
Adults range from 1mm
to 25 m in length
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Anterior scolex solely for attachment to the host's gut
a string of proglottids, each of which possesses both
male and female organs