Acoelomorpha, Platyzoa, and Mesozoa

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Acoelomorpha, Platyzoa, and Mesozoa

• 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.

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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
• 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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Ecdysozoa Lophotrochozoa

• Clades within Protostomia
• Modern molecular phylogenies have grouped
  acoelomate and coelomate taxa together within the
  protostomes.
• Protostomes now divided into two large clades
  Ecdysozoa and Lophotrochozoa
• Ecdysozoa possess a cuticle that is molted as
  their bodies grow
• Lophotrochozoa share either an odd horse-shoe
  shaped feeding structure, the lophophore or have
  unique larval form called the trochophore

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Ecdysozoa Lophotrochozoa

• Trochophore larvae
• Minute, translucent, and roughly top-shaped
• Have a prominent circlet of cilia and sometimes
  one or two accessory circlets
• Occur in the early development of other marine
  members of Annelida and Mollusca and are assumed
  to be the ancestors of such groups
• Trochophore-like larvae also occur in some
  Platyhelminthes, Nemertean, Echiura and
  Sipunculida groups.

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Clade Platyzoa

• Clade Platyzoa is a unique group of
  lophotrochozoan protostomes that contain
  Platyhelminthes, Gastrotricha and Gnathifera.

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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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Pseudocoelomates

• Pseudocoelomates have a body cavity (the
  pseudocoel) between the gut (derived from
  endoderm) and body wall (derived from mesoderm).
• Derived from the blastocoel.

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Advantages of a Body Cavity

• A body cavity, pseudocoel or coelom, has several
  advantages. It provides
• Greater freedom of movement.
• Space for development of organ systems.
• A simple means for circulation of materials
  around the body.
• Storage place for waste products.
• A hydrostatic organ.

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Pseudocoelomates

• Pseudocoelomates do not form a clade.
• Some are part of superphylum Lophotrochozoa,
  others are in superphylum Ecdysozoa.
• All share the pseudocoelomate body plan.

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Lophotrochozoa (10 Phyla)

• Ancestors possessed complex cuticular jaws
  Clade Gnathifera
• Gnathostomulida
• Micrognathozoa
• Rotifera
• Acanthocephala
• 6 other lophotrochozoan phyla
• Gastrotricha
• Tiny aquatic animals that may be closely related
  to gnathiferans
• Molecular characteristics place the following
  with Lophotrochozoa
• Cycliophora
• Entoprocta
• Ectoprocta
• Brachiopoda
• Phoronida

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Phylum Gastrotricha

• Gastrotrichs appear similar to rotifers, but
  without the ciliated corona and have a bristly
  looking body.
• Members of the phylum Gastrotricha are
  pseudocoelomate and have three embryonic germ
  layers (triploblastic).
• Complete digestive system.
• Hermaphroditic or parthenogenetic.

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Clade Gnathifera

• Possess small cuticular jaws with a homologous
  microstructure.
• Numbers of pairs of jaws vary.
• Gnathostomulida, Micrognathozoa, and Rotifera are
  tiny, free-living, aquatic animals.
• Acanthocephalans are worm-like endoparasites
  living as adults in fish or other vertebrates.

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Clade Gnathifera

• Rotifera and Acanthocephala
• Presumed sister taxa.
• Form a clade called Syndermata.
• Have eutelic syncytial epidermis.
• Constant number of nuclei.
• Grouping is controversial.

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Phylum Gnathostomulida

• Phylum Gnathostomulida includes the jaw worms.
• Very small - lt2mm.
• Live in interstitial spaces of fine coastal
  sediments.
• Can endure low O2.

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Phylum Gnathostomulida

• Feed by scraping bacteria and fungi from the
  substratum with a pair of jaws on the pharynx.
• Acoelomate
• Sexual stages include males, females, and
  hermaphrodites.
• Fertilization is internal.

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Phylum Micrognathozoa

• Micrognathozoans are tiny animals that live
  interstitially (between sand grains).
• Body consists of a two-part head, a thorax, and
  abdomen with short tail.
• Move using cilia and have a unique ventral
  ciliary adhesive pad that produces glue.
• Three pairs of complex jaws.

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Phylum Micrognathozoa

• Simple gut
• Anus opens to outside only periodically.
• Reproductive system is not well understood
• Only female reproductive organs have been
  identified.
• May reproduce parthenogenetically.
• Cleavage and subsequent development have not been
  studied.

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Phylum Rotifera

• Members of the phylum Rotifera are
  pseudocoelomate and have three embryonic germ
  layers (triploblastic).
• Complete digestive system.

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Phylum Rotifera

• Dioecious (separate sexes) but some species
  parthenogenetic (females produce diploid eggs).
• Some are parthenogenetic during part of the year,
  depending on environmental conditions.
• Thick shelled eggs that can withstand harsh
  conditions are sometimes produced.

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Phylum Rotifera

• Rotifers have a ciliated crown, the corona, that
  is characteristic of the phylum.

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Phylum Rotifera

• Rotifers come in a wide range of colors and
  shapes.
• Shapes often correspond to lifestyle (floaters,
  swimmers, sessile).
• They may be individual or colonial.
• Mostly freshwater.
• Benthic and pelagic forms.

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Phylum Acanthocephala

• All spiny-headed worms (Phylum Acanthocephala)
  are parasites in the intestines of vertebrates.
• Over 1100 species known.
• Occur worldwide and parasitize fish, birds, and
  mammals.
• Larvae develop in crustaceans or insects.

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Phylum Acanthocephala

• Proboscis has rows of recurved spines that
  penetrate and may rupture host intestines.
• Proboscis with hooks can be inverted into a
  proboscis receptacle by retractor muscles.

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Phylum Acanthocephala

• Body somewhat flattened.
• About 80 of tegument is a lacunar system of
  fluid-filled canals that may distribute nutrients
  and remove wastes from muscles.
• No heart - function provided by lacunar fluid.
• Both longitudinal and circular body wall muscles
  are present.

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Phylum Acanthocephala

• No respiratory system.
• Protonephridia with flame cells, if present,
  perform excretory functions.
• Nutrients are absorbed across the tegument, which
  bears some enzymes - no digestive tract.
• Dioecious
• No species normally parasitizes humans.

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Phylum Acanthocephala

• Acanthocephalans penetrate the intestinal wall
  with spiny proboscis.
• Remarkably little inflammation on host wall, but
  pain of infection is intense.
• Larval acanthors burrow through beetle intestine.
• Develop into juvenile cystacanths in the insect
  hemocoel.
• Pigs become infected by eating grubs.

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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.

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Phylum Mesozoa

• 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.
• Mesozoans are identified as lophotrochozoan
  protostomes based on molecular data, but are not
  placed in Platyzoa.