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The Triploblasitc, Acoelomate Body Plan
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Title: The Triploblasitc, Acoelomate Body Plan
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The Triploblasitc, Acoelomate Body Plan
- Zoology Chapter 10
- Biology Chapter 27
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Acoelmates, Pseudocoelomates, and Coelomates
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All animals in this chapter are
- Triploblastic have three primary germ layers
- Acoelomate without a coelom
- Classified into three phyla
- Phylum Platyhelminthes (flatworms)
- Phylum Nemertea (unsegmented)
- Phylum Gastrotricha (bottom dwellers)
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Phylum Platyhelminthes
- aka 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
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Flatworm Characteristics
- Bilateral Symmetry
- Cephalization (have a so
- called head)
- Mesoderm tissue includes a
- loose tissue (parenchyma)
- that fills spaces between
- specialized tissues, organs,
- and body wall.
- May provide skeletal support, nutrient storage,
motility, transport of materials, oxygen storage,
etc.
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Form and Function in Flatworms
- Most cells are close to external environment ?
materials can pass easily into and out of their
bodies (via diffusion) - Rely on diffusion for respiration, excretion, and
circulation - Other functions vary among species (free-living
or parasitic)
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4 Classes of Platyhelminthes
- Free-living flatworms
- Class Turbellaria
- Parasitic Species
- Class Monogenea
- Class Trematoda
- Class Cestoidea
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Class Turbellaria
- 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
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Class Turbellaria cont.
- 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
appear
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Feeding
- Carnivores (small invert.),
- scavengers (dead), some
- herbivores (algae)
- Sensory cells (chemoreceptors) on their heads
help detect food far away - Digestive cavity has a single opening (or mouth)
through which food and waste pass
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Feeding cont.
- Pharynx muscular
- tube that extends
- out of the mouth and
- pumps food into the
- digestive cavity or gut
- highly branched gut transports food to all parts
of the body (this is lacking in many parasitic
species)
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Locomotion
- 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
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Locomotion cont.
- Muscle cells controlled by the nervous system
allow them to twist and turn so that they are
able to react rapidly to environmental stimuli - Dorsoventral muscles essential for maintaining
flatness (ie-for diffusion)
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Exchanges with Environment
- Do not have respiratory organs ? done via
diffusion - Respiratory gases (CO2 and O2)
- Metabolic wastes (ammonia)
- Depends on environment
- Marine osmotic equilibrium
- Freshwater hypertonic
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Excretion
- Protonephridia is a network of fine tubules for
excretion of metabolic wastes. - Flame cells are ciliated and induce currents to
push fluids through tubules.
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- Tubules eventually merge and open to the outside
of the body wall through a minute opening called
a nephridiopore.
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Nervous System Sense Organs
- Subepidermal nerve plexus (resembles cnidarians
but depends on species) - Mechanoreceptor (excited by pressure) at anterior
end detects body position(due to gravity) - Cerebral ganglia more centralized nerve
- net (brain)
- Longitudinal nerve cords
- ladderlike 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
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Reproduction
- Asexual reproduction
- Budding or fission
- Regeneration
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Reproduction cont.
- Can reproduce either asexually or sexually
- Asexual planarian detaches its tail end and
each half regrows the lost parts (referred to as
zooids). - Sexual each Planaria gives and receives sperm.
- Benefits to sexual over asexual?
- Have 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.
- Can also reproduce by regeneration
(fragmentation) - If it is cut into two halves, both halves may
become two new Planaria.
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Turbellariareview
- What type of
- symmetry?
- How do they
- feed?
- What type of
- sensory organs?
- How do they
- reproduce?
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Parasitic worms
- Class Monogenea
- Class Trematoda
- Class Cestoidea
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Class Monogenea
- External parasites (ectoparasitic)
- Commonly found on the gills, skin or fins of
fishes and lower aquatic invertebrates - Have a series of hooks that enable them to attach
while feeding - Have one generation in their life cycle from egg
to adult
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Class Trematoda
- 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)
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Subclass Digeneans
- Life cycle involves two - four hosts and a number
of developmental stages, including two types of
free-living larvae (most complex in the animal
kingdom) - The definitive (final) host of adult is always a
vertebrate - Snails are common intermediate hosts
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- Eggs reach freshwater and Miracidium (ciliated
larva) swims out - Finds host (snail)
- Penetrates snail, loses cilia, develops into a
sporocyst (contain embryonic cells) - Develops into daughter sporocysts (hundreds can
form from one miracidium) - Hundreds of next larval stage are produced
(cercariae) - Cercariae leave the snail and find 2nd or final
host - Penetrates host and becomes a Metacercaria
- When the definitive host eats the 2nd
intermediate host, it becomes an adult
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The species that infect humans can be divided
into groupsSchistosomes non-Schistosomes
- Dicrocoelium dendriticum and Opisthorchis sp.
liver flukes of mammals - blood flukes, Schistosoma spp. are among most
widespread and serious parasites of humans
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Schistosomiasomes (blood flukes)
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Class Cestoidea
- 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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- The body consists of an anterior scolex solely
for attachment to the host's gut and a string of
proglottids, each of which possesses both male
and female organs
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Phylum Nemertea
- aka ribbon worms
- or proboscis worms
- 900 species
- More closely related to other acoelomates than to
any flatworm - Longest of any invertebrate (30 m but 2x as long
when fully extended)
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Phylum Nemertea
- Unsegmented
- Proboscis tipped
- with barbs to capture prey (see it in action!)
- Complete digestive tract with an anus
- Closed circulatory system (w/o a heart)
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Phylum Gastrotricha
- 500 species
- Bottom dwellers in both marine freshwater
- Range from 0.1 mm to 4 mm
- Dorsal contains scales, bristles,
- or spines
- Have a forked tail
- Development is direct
- (there is no larval stage)
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