Flatworms, Mesozoans, and Ribbon Worms

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Flatworms, Mesozoans, and Ribbon Worms

• Chapter 14

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Bilateria

• Most animals have bilateral symmetry.
• The vast majority of animal species belong to the
  clade Bilateria, which consists of animals with
  bilateral symmetry and triploblastic development.

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Bilateral Symmetry

• Radially symmetrical animals have the world
  coming at them from all directions.
• They can catch prey coming from any direction.
• Animals that begin to move about actively seeking
  food need a different body organization.
• Distinct head end with sensory structures.
• Cephalization

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Bilateral Symmetry

• Animals with bilateral symmetry have a distinct
  head end and can be divided into right and left
  halves.

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Acoelomate Bilateral Animals

• Animals that have no space between their gut and
  body wall are said to be acoelomate.
• These animals are also triploblastic they have
  three embryonic germ layers.
• Organ-system level of organization more
  division of labor among their organs.

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Acoelomates

• Although flatworms undergo triploblastic
  development, they are acoelomates.

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Acoelomates

• These acoelomate phyla are protostomes and have
  spiral cleavage.
• Most have determinate cleavage.
• These are the simplest animals with an excretory
  system.
• Acoelomate phyla belong to the superphylum
  Lophotrochozoa

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Phylum Acoelomorpha

• Group contains 350 species.
• Members were formerly in Class Turbellaria within
  phylum Platyhelminthes Small flat worms less than
  5 mm in length.
• Typically live in marine sediments few are
  pelagic.
• Some species live in brackish water.
• Most symbiotic but some parasitic.
• Have a cellular ciliated epidermis.
• Parenchyma layer contains small amount of ECM and
  circular, longitudinal, and diagonal muscles.

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Phylum Acoelomorpha - Digestion and Nutrition

• Incomplete digestive system - no anus.
• In many acoels, the gut and pharynx are absent.
• Phagocytotic cells digest food intracellularly
  when food is passed into temporary spaces.

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Phylum Acoelomorpha - Reproduction

• Monoecious
• Female produces yolk-filled, endolecithal eggs.
• Following fertilization some or all cleavage
  events produce a duet-spiral pattern of new
  cells.
• May be a defining character for acoelomorphs.

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Phylum Platyhelminthes

• Members of phylum Platyhelminthes live in marine,
  freshwater, and damp terrestrial habitats.

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Phylum Platyhelminthes

• Flatworms are flattened dorsoventrally and have a
  gastrovascular cavity.
• Extracellular digestion.
• Undigested food is egested through the pharynx.

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Phylum Platyhelminthes

• The osmoregulatory system consists of
  protonephridia (excretory or osmoregulatory
  organs closed at the inner end) with flame cells.
• Most metabolic wastes removed by diffusion across
  the body wall.

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Phylum Platyhelminthes

• The nervous system consists of a ladder-like
  network of nerves and a bilobed brain.
• Many have large ocelli light sensing organs.

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Phylum Platyhelminthes

• Many can reproduce asexually as well as sexually.
• Asexual reproduction via fission.
• Sometimes the new individuals remain attached
  chains of zooids.
• Monoecious

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Taxonomy

• Flatworms (phylum Platyhelminthes) are divided
  into four classes
• Class Turbellaria ex. Planaria
• Not monophyletic
• Class Trematoda parasitic flukes
• Class Monogenea parasitic monogenetic flukes
• Class Cestoda - tapeworms

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Phylum Platyhelminthes
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Class Turbellaria

• Turbellarians are nearly all free-living and
  mostly marine.

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Class Turbellaria

• The best-known turbellarians, commonly called
  planarians, have light-sensitive eyespots and
  centralized nerve nets.

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Class Trematoda

• Trematodes live as parasites in or on other
  animals.
• They parasitize a wide range of hosts.

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Class Trematoda

• Subclass Digenea, digenetic flukes, have a
  complex life cycle with a mollusc (snail) as the
  first host and a vertebrate as the final, or
  definitive, host.

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Class Monogenea

• All monogeneans are parasites.
• Often found in the gills or external surfaces of
  fishes.

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Class Cestoda

• Tapeworms (Class Cestoda) are also parasitic and
  lack a digestive system.
• The scolex is equipped with suckers and hooks for
  attachment to the host.
• Each proglottid contains a set of reproductive
  organs.

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Class Cestoda

• Cestodes usually require at least two hosts.
• Adult cestodes are parasites in the digestive
  tracts of vertebrates.

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Phylum Mesozoa

• Phylum Mesozoa is considered a missing link
  between protozoa and metazoa.
• Have a simple level of organization.
• Minute, ciliated, and wormlike animals.
• All live as parasites in marine invertebrates.
• Most composed of only 20 to 30 cells arranged in
  two layers.
• Layers are not homologous to germ layers of other
  metazoans.
• Two classes, Rhombozoa and Orthonectida, are so
  different that some authorities place them in
  separate phyla.

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Phylum Mesozoa

• Rhombozoans live in kidneys of benthic
  cephalopods.
• Adults called vermiforms and are long and
  slender.
• Inner, reproductive cells give rise to vermiform
  larvae.
• When overpopulated, reproductive cells develop
  into gonad-like structures producing male and
  female gametes.
• Larvae are shed with host urine into the seawater.

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Phylum Mesozoa

• Orthonectids parasitize a variety of
  invertebrates.
• Reproduce sexually and asexually.
• Asexual reproduction consists of a multinucleated
  mass called a plasmodium.

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Phylogeny of Mesozoans

• Some consider these organisms primitive flatworms
  and place them in phylum Platyhelminthes.
• Molecular evidence groups them with flatworms in
  superphylum Lophotrochozoa.
• However, molecular phylogeny that included an
  orthonectid and two species from a rhombozoan
  subgroup, the dicyemids, did not show members of
  the two classes to be sister taxa.
• The phylum may not be monophyletic.

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Phylum Nemertea

• Ribbon worms, phylum Nemertea, use a proboscis to
  capture prey.
• Almost completely marine.
• Active predators.
• General body plan similar to turbellarians.

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Phylum Nemertea

• An anus is present providing these worms with a
  complete digestive system.
• Nermeteans are the simplest animals to have a
  closed loop blood-vascular system.

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Phylogeny

• A planuloid ancestor (like the planula larva of
  cnidarians?) may have given rise to a branch of
  descendents that were sessile or free floating
  and radial Cnidaria.
• Another branch acquired a creeping habit and
  bilateral symmetry
• Bilateria.