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(click on pic for source)

    Parazoa: The Sponges

    Sponges are multicellular proto-animals
    • composed of diverse, cooperative cells
    • lacking true tissues
    • lacking a true plane of symmetry
    • that obtain food via filter feeding
    • are characterized by tiny holes throughout the thallus

    Porifera is Latin for "bearer of holes".
    (Latin por means "hole" and fer means "to bear".

    Most of the 9000+ sponge species are marine,
    but about 150 species live in fresh water.

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    Sponge Phylogeny and Evolution

    Sponge phylogeny is complex and still being unraveled.

    Traditionally, sponges have been divided into three major groups:

    • Calcarea - The Calcareous Sponges
    • Hexactinellida - The Glass Sponges
    • Demospongiae - The Bath Sponges

    ...but these groupings (and even "Parazoa" itself) may not be monophyletic.

    All sponges have cells resembling extant choanoflagellate protists.

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    The Architecture of a Sponge

    All sponges have four specialized cell types.

    • choanocytes - flagellated collar cells generate water current
    • pinacocytes - form the outer pinacoderm
    • porocytes - tubular cells form incurrent pores
    • archaeocytes/amoebocytes - roving scavenger cells
      that facilitate digestion and feed other cells

    Digestion is intracellular, as there is no gut.

    In primitive condition, the sponge body has three layers that are not true tissues:

    • external, epithelium-like pinacoderm
    • internal, flagellated choanoderm
    • gelatinous mesenchyme (=mesohyl) sandwiched between

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    The Folding of a Sponge

    The simplest sponges form a radial tube
    with external pinacoderm and external choanoderm.

    More derived sponges exhibit increased "folding".

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(source: Wikimedia Commons)

    The Cells of a Sponge

    Sponges are composed of four types of
    pluripotent, cooperative cells.

    • Choanocytes form the inner choanoderm.
    • Pinacocytes form the outer pinacoderm.
    • Porocytes form the incurrent pores (ostia)
    • Amoebocytes roam the mesenchyme

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    Skeletal Elements: Spicules and Proteins

    Specialized amoebocyte cells secrete skeletal elements
    • sclerocytes
        secrete spicules (silica or CaCO3) --->
    • spongocytes
        secrete proteinaceous spongin fibers
    • collenocytes
        secrete proteinaceous collagen fibers

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    Sponge Cells: Pluripotent and Creepy

    If you disrupt the cells of a sponge, it can re-aggregate
    to form a complete, new sponge.

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    Meet the Sponges: Calcarea

    The calcareous sponges are the most primitive sponges.

    • Spicules are primarily alcium carbonate (CaCO3)c.
    • All species are marine.
    • They are typically small and pale-colored.
    • They are often asconoid in form.

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(source: McGraw Hill Publishing)

    Meet the Sponges: Hexactinellida

    Glass sponges are spectacular.

    • All species are marine.
    • Six-rayed silica spicules form a glass lattice infrastructure.
    • Suspended between the spicules is a coenocytic syncytium:
      • consists of many fused amoebocyte pseudopods
      • has multiple nuclei
      • forms chamber that opens to the spongocoel
      • largest continuous syncytium known in the Metazoa!

      Some species have an interesting commensal relationship with shrimp.

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    Meet the Sponges: Demospongiae

    The demosponges are the most diverse group.
    More than 95% of all sponge species are demosponges,
    including the familiar "bath sponge".
    • The spicules may be silica and/or calcium carbonate
    • Most species are leuconoid in form.
    • Most species are marine, but some live in fresh water.
    • A proteinaceous network of flexible spongin fibers provides structural support
    • Some species with 100% spongin skeletons are harvested as bath sponges

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Eumetazoa: Placozoa

Placozoa: The Simplest Animals

Placozoans are of uncertain evolutionary affinity.

They are little more than a jelly-like plate of interdependent cells
exhibiting the beginnings of radial symmetry.
There is only one recognized extant species, Trichoplax adhaerens.

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(photo by Tara Sutherland)

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    Eumetazoa: Radiata

    Animals currently classified as Radiata

      • are radially symmetrical
      • lack cephalization
      • have an oral --> aboral body axis
      • are diploblastic
        • ectoderm (becomes the epidermis in the adult)
        • endoderm (becomes the gastrodermis in the adult)
        • mesogloea (gelatinous middle layer is not true mesoderm)

    This group includes Cnidaria and Ctenophora

(click on pic for source)

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    Cnidaria

    There are approximately 10,000 species of described,
    named cnidarians in four Classes.


    • Anthozoa - sea anemones, corals, sea pens, sea pansies
        • polyp is the dominant life cycle stage

    • Hydrozoa - hydras, Portuguese Man o' War
        • most alternate between polyp and medusa

    • Scyphozoa - jellyfish
        • medusa is the dominant life cycle stage

    • Cubozoa - box jellies (some highly venomous!)
        • medusa is the dominant life cycle stage

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      The Beauty of Jellies

      Sea jellies may be one of the few marine organisms
      that will benefit from the acidification of the oceans
      due to anthropogenic climate change.

      Their populations are growing as those of other
      marine invertebrates decline.

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    (click on pic for source)

      Simple Beginnings

      The cnidarian larva is a ciliated, free-swimming gastrula called a planula.

      More sophisticated than it looks, the planula can sense light and move appropriately.

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      Polyp vs. Medusa

      Many cnidarian species are dimorphic: alternating between

      • polyp phase (asexually reproducing, feeding form)

      • medusa phase (sexually reproductive form)

      There is a mouth derived from the blastopore, but no anus.
      What goes in comes out the same way, after digestion.

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      The Knide in the Kname

      The phylum name derives from the Greek knide, meaning "nettle".

      The presence of unique cells called cnidoblasts (= cnidocytes)
      is a synapomorphy uniting all cnidarians.

      Each cnidocyte contains a stinging capsule, the nematocyst,
      which everts when the cnidocil trigger is brushed.

      The nematocyst may be sticky or barbed,
      and is often equipped with powerful toxins
      that quickly subdue prey.

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    Radiata: Ctenophora (~100 species)

    There about 100 known species of comb jellies.
    • They are pelagic carnivores
    • Paired, longitudinal rows of cilia provide locomotion.
    • The cilia are joined at the tips, giving them a comblike appearance.
    • They lack stinging cells
    • Their tentacles are equipped with many multicellular colloblasts
      that produce sticky adhesive for trapping prey.

    Until recently, ctenophores were believed to have a mouth but no anus.
    Newly discovered anal pores have stunned evolutionary biologists.

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      Disappearing Act

      One reason we know of only 100 species of comb jellies is that they are nearly invisible in their environment.

      Their tissues have the same refractive index as water.

      Only when the light catches them just the right way do the proteins in their tissues refract light in sometimes spectacular ways.

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      Eumetazoan Diversity: Bilateria

      The Bilateria
      • comprise a vast, diverse assemblage
      • are bilaterally symmetrical
      • exhibit distinct cephalization
      • have a craniocaudal (anteroposterior) body axis
      • have a dorsoventral body axis
      • are triploblastic
        • ectoderm (becomes the epidermis in the adult)
        • endoderm (becomes the gastrodermis in the adult)
        • mesoderm (derived from endoderm)

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    Just to remind us of where we are...


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    Click on pic for source and more information.

      Bilateria: Xenacoelomorpha (~ 370 species)

      The acoel flatworms are the simplest bilateral animals.
      Several studies suggest that they may be basal to all other bilaterians.

      Molecular data indicate that acoel flatworms are descended
      from the earlist offshoots of the first bilaterally symmetrical animal.

      The blastopore becomes neither the mouth nor the anus.

      Instead, the blastopore closes during development,
      and a new opening becomes the mouth.

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      What's in a Worm?

      Acoels
      • are strictly marine and brackish
      • are found in all the earth's oceans
      • exhibit true bilateral symmetry
      • exhibit cephalization (but no brain)
      • range in size from 2mm - 15mm
      • Have simple sense organs:
        • In some species, light detecting ocellus (plural = ocelli) may be present.

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    Waminoa sp. (orange) on bubble coral (white) (photo by Samuel Chow, Wikimedia Commons)

      Answer: Not Much

      Acoels have some traits reminiscent of a simple planula:
      • The primordial "gut" is not composed of true endoderm
        • Unlike a true intestine, the acoel gut is not lined with epithelial cells.
        • The integument is composed of epithelial cells.
        • mesenchyme (mesogloea with cellular components) is present.
      • Acoels lack ovaries or testes.
      • Meiosis occurs in specialized diploid cells.
      • A single individual can produce both ova and sperm.
      • They are thus hermaphroditic

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      Who ARE These Guys?

      Recent molecular data revealed the presence of microRNA sequences
      that are found elsewhere only in the Deuterostomia.

      Acoels may be basal deuterostomes.

      Or protostomes may have lost that microRNA.

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    (click on pic for source and more information)

      Eumetazoan Diversity - Bilateria: Protostomia

      Primitively, most protostomes exhibit

      • a blastopore that becomes the mouth
      • spiral cleavage, illustrated to the left
      • determinate cleavage (blastomere fate is fixed early in development)
      • schizocoely to produce a true body cavity (coelom)

      There are two major lineages: Lophotrochozoa and Ecdysozoa.

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      Protostomia: Lophotrochozoa

      Lophotrochozoa includes (for the moment)
      • Platyhelminthes (free-living flatworms, flukes, and tapeworms)
      • Syndermata (rotifers and acanthocephalans)
      • Lophophorata (bryozoans, brachiopods, phoronid worms)
      • Nemertea (ribbon worms)
      • Annelida (segmented worms)
      • Mollusca (mollusks)

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      Protostomia: Ecdysozoa

      Ecdysozoa includes
      • Kinorhyncha (spiny-crown worms)
      • Priapulida (penis worms)
      • Chaetognatha (arrow worms)
      • Nematorpha (horsehair worms)
      • Nematoda (roundworms)
      • Onychophora (velvet worms)
      • Tardigrada (water bears)
      • Arthropoda (joint-footed animals)

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    (click on pic for source)

      Meet the Lophotrochozoa

      Lophotrochozoa is named for two characters exhibited by its members
      • the lophophore, a ciliated crown of tentacles surrounding the mouth
      • the trochophore, a distinctive larva

      Some species have lophophores.
      Other species have trochophores.
      Some have neither.
      Go figure.


    (click on pic for source)

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      Platyhelminthes: The Flatworms

      Primitively, the flatworms are characterized by
      • acoelomate body plan
      • more distinct cephalization than acoels
      • more complex sense organs
      • simple intestine (one opening serves as both mouth and anus)
      • excretory system
      • hermaphroditism

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      Peripatetics and Parasites

      Platyhelminthes includes

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      Platyhelminth Architecture

      In the Platyhelminthes, organ systems become better developed:
      • integumentary system
      • digestive system (primitive: mouth, but no anus)
        • well developed in turbellaria
        • reduced in flukes
        • lost in tapeworms
      • nervous system
        • well developed in turbellaria
        • reduced in flukes
        • even more reduced in tapeworms
      • sense organs
        • well developed in turbellaria
        • reduced in flukes
        • vestigial in tapeworms
      • muscular system
        • well developed in turbellaria
        • well developed in flukes
        • vestigial in tapeworms
      • reproductive system (all hermaphroditic)
      • excretory system
        • Simple, tubular protonephridial system composed of anucleate flame cells concentrate & excrete nitrogenous waste.

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      But Flatworms Don't Have It All

      Platyhelminths lack:
      • skeletal system
      • circulatory system
      • respiratory system
      • immune, lymphatic or endocrine systems

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      Platyhelminthes: Free-living flatworms
      (> 4500 species)

      Free-living platyhelminths are commonly called planarians.

      Compared to parasitic platyhelminths, planarians have

      • the highest degree of cephalization among platyhelminths
      • the best developed nervous and other organ systems
        • chemoreceptors are borne on flaplike extensions, called auricles
        • (They have nothing to do with hearing.)
        • Photoreceptors are simple ocelli (singular - ocellus).
        • Statocysts (gravity sensing cells) occur near the cerebral ganglion.

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      Pretty But Deadly

      Some marine polyclad flatworms exhibit
      aposematic (warning) coloration and are highly toxic.

      And some of them enjoy a bit of penis fencing now and then.

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    (click on pic for source)

      Platyhelminthes: Neodermata - Parasitic Flatworms

      All species in this clade are parasitic.

      In some species, more than one intermediate host
      is required for the full life cycle to be completed.

      • definitive host - organism in which the adult parasite resides
      • intermediate host - organism in which various larval/developmental stages
        of the parasite exists until they are passed to the definitive host

      Most parasites are host-specific, but some can inhabit
      more than one different species of definitive host.

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    (click on pic for source)

      Parasite Transmission

      Transmission of parasites (or pathogens) can be

      • horizontal
          from one individual to another via direct or environmental contact

      • vertical
          from parent to offspring, at or before birth

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    (click on pic for source)

      Tegument Affords Protection

      Parasitic flukes and tapeworms have a protective tegument
      external to the epidermis.

      It is formed by nonciliated, cytoplasmic extensions of mesenchymal cells.

      It affords protection from host's enzymes and immune system.

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      Trematoda - The Flukes (~10,000 species)

      Flukes
      • are entirely parasitic
      • often have complex life cycles with multiple intermediate hosts
      • have reduced sensory systems and other organ systems

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      Fluke Life Cycles

      Flukes can inhabit any number of host tissues, including
      • mouth and pharyngeal mucosa
      • internal organs
      • blood
      • other connective tissues
      • you name it

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    (click on pic for source)

      Cestoda - The Tapeworms (~6000 species)

      Like flukes, tapeworms are entirely parasitic,

      Their organ systems are even more reduced than those of flukes.

      • nervous system is vestigial
      • digestive system has been lost
      • Transport of nutrients, oxygen, and waste takes place across the integument.

      To make things even simpler, all are hermaphroditic.

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      Tapeworm Anatomy

      Tapeworms are streamlined parasitic machines.
      • The head (scolex) is equipped with hooks or suckers.
      • Segments (proglottids) and grow from the scolex.
      • Early proglottids contain both ovaries and testes.
      • Tapeworms can self-fertilize.
      • Older proglottids contain hundreds of eggs.
      • These are shed with the host's feces.

        Here's a live passenger:

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      Tapeworm Life Cycle

      • Ingested eggs hatch in the gut and larvae burrow through the intestinal wall into the body cavity, muscle and connective tissue, or into the bloodstream.

      • Each larva then encysts to form a cysticercus in the intermediate host.

      • These lie dormant until intermediate host is eaten by a definitive host.

      • The cysticercus pops its head out of the cyst, attaches to the intestinal wall of the definitive host, and grows into a new tapeworm.

      • Lather, rinse, repeat.

      • Yum!

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      Cysticercosis

      A unintended animal can sometimes accidentally become an intermediate host

      • A human eats vegetable matter contaminated with tapeworm eggs

      • Larvae hatch and migrate to connective tissue, muscle, CNS and encyst

      • The result: potentially life-threatening cysticercosis

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      Go forth to the sushi bar.

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