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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1. Eggs reach freshwater and Miracidium (ciliated
   larva) swims out
2. Finds host (snail)
3. Penetrates snail, loses cilia, develops into a
   sporocyst (contain embryonic cells)
4. Develops into daughter sporocysts (hundreds can
   form from one miracidium)
5. Hundreds of next larval stage are produced
   (cercariae)
6. Cercariae leave the snail and find 2nd or final
   host
7. Penetrates host and becomes a Metacercaria
8. 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)