Instructions for printer-friendly copy.

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    Ecdysozoa: The Molting Animals

    Ecdysozoan animals are named for ecdysis,
    the molting/shedding of the external cuticle/exoskeleton
    as the animal grows.

    We will visit these Ecdysozoan taxa:

    • Priapulida - The Penis Worms
    • Nematomorpha - The Horsehair Worms
    • Nematoda - The Roundworms
    • Onychophora - The Velvet Worms
    • Tardigrada - The Water Bears
    • Arthropoda - The Joint-legged Animals

      ...all of which undergo ecdysis as they grow.

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    Chitin

    The main structural component
    of the ecdysozoan cuticle is chitin.

    Chitin is a polymer of the a glucose derivative,
    N-acetyle glucosamine.

    It is the second most abundant biopolymer on earth.

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    Ecdysozoan Phylogeny

    Molecular data support the monophyly of Ecdysozoa.
    Within the taxon, however, relationships remain uncertain.

    One hypothetical phylogeny is shown here.

    Ecdysozoa includes both

    • pseudocoelomates
    • coelomates

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    Convergent Body Cavities

    Remember that the pseudocoelom,
    a persistent blastocoel lined only on the parietal side
    by mesoderm, evolved independently in many Bilaterian lineages.

    The ecdysozoan pseudocoelom is not homologous
    to the pseudocoelom in other taxa.

    It may even have evolved several times
    within the Ecdysozoa.

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(click on pic for source)

    Priapulida - The "Penis Worms"

    There are only 20 species in this group of marine predators,
    named for Priapus, the Greek god of fertility.

    Penis worms...

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    Nematomorpha - The Horsehair Worms (~360 species)

    Mostly freshwater, but a few marine, these long, unsegmented worms are
    • free-living as adults
    • parasites on arthropods as larvae

    The large pseudocoelom houses a hydrostatic skeleton.

    Their whiplike motion is generated via contraction only of longitudinal muscles.

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    Nematoda - The Roundworms
    (~15,000 known species)

    Nematodes are the most numerous, ubiquitous animals on earth.

    Thousands of individuals may inhabit a single rotting apple.

    One gravid female may contain 27 million eggs
    and lay 200,000 of them in one day.

    They are in you and on you and all around you.

    Nematodes may be...

    • beneficial members of soil communities
    • commensal
    • parasites of animals and plants.

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    You're Not Alone in There

    Nematodes are characterized by:
    • a large pseudocoelom housing a hydrostatic skeleton
    • no metamerism
    • only longitudinal muscles being functional
    • a characteristic sinusoidal swimming motion
    • a simple excretory system
    • lack of circulatory or respiratory systems

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    Parasitic Nematodes

    Relatively few nematode species cause disease.

    But the effects of parasitic nematode effects can range
    from the annoying to the catastrophic.

    Enterobius sp. ("pinworms") are relatively common intestinal parasites in humans and other mammals.

    They are generally just annoying and itchy.

    And they can be passed among mammal species.
    Yay!

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(click on pic for source)

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    Parasitic Nematodes: Heart Worm

    Dirofilaria immitis, the canine heartworm
    can be life-threatening.


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    Parasitic Nematodes: Elephantiasis

  • Wuchereria bancrofti, Brugia malayi, and B. timori
    can cause lymphatic filariasis ("elephantiasis").

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    Coelomate Ecdysozoans

    Three phyla of Ecdysozoans are
    • coelomate
    • equipped with paired walking appendages
      on each segment

    These are

    • Onychophora (velvet worms)
    • Tardigrada (water bears)
    • Arthropoda (the joint-legged animals)

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    Onychophora - The Velvet Worms

    These small, tropical predators are semi-fossorial, burrowing in leaf litter.

    Velvet worms are characterized by

    • distinct metamerism
    • paired, chitinous appendages on each segment
    • open circulatory system
    • haemocoel as main body cavity
    • coelom reduced to a gonocoel and pericardium.

    Velvet worms give birth to and care for live young.

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The grasshopper illustrates the open circulatory system.

    Circulatory Systems: Open or Closed?

    In a closed circulatory system, a heart pumps blood
    enclosed at all times within vessels of different size
    and wall thickness.

    In an open circulatory system a heart pumps hemolymph
    into a hemocoel body cavity where it sloshes around
    and bathes tissues and organs in dissolved nutrients and gases.

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(click on pic for source)

    Tardigrada - Water Bears

    Tardigrades are microscopic (~1mm) fluid-feeders.
    They live in freshwater, marine, and moist terrestrial habitats
    where they feed on the juices of other organisms.

    Tardigrades are characterized by...

    • distinct metamerism
    • two chitinous, clawed appendages per segment
    • an open circulatory system
    • a haemocoel as main body cavity
    • a coelom reduced to a gonocoel and pericardium.

    According to Johann August Ephraim Goeze,
    who described them in 1773,
    they lumber around like tiny, aquatic bears

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(click on pic for source)

    Tough as a Tardigrade

    Tardigrades have an amazing ability to survive
    physical conditions that would kill any other organism.
    • extreme heat and cold
    • nearly 100% desiccation
    • vaccuum conditions (zero oxygen!)
    • desiccation for hundreds of years

    Investigators are studying tardigrade cryptobiosis,
    a physiological state in which metabolic processes
    are so reduced as to be undetectable.

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(click on pic for source)

    Arthropoda
    The Joint-legged Animals

    The arthropods comprise 5 - 10 million species,
    rivaled only by nematodes for sheer numbers.

    Arthropods inhabit every conceivable environment, from open ocean, to the bodies of fungi, plants,
    and animals, including your eyelash follicles.

    Our understanding of arthropod evolutionary relationships has undergone major revision
    over the past decade or so.

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(Click on pic for source)

Two major hypotheses:

  • Paradoxopoda (supported by morphological characters)
  • Mandibulata (supported by mtDNA data)

    Arthropod Evolution

    Four major clades are likely monophyletic:
    • Chelicerata
    • Myriapoda
    • Crustacea
    • Hexapoda

    But the evolutionary relationships among
    these four taxa remain uncertain.

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    Arthropod Architecture

    Arthropods are characterized by...
    • jointed limbs
    • a tough, articulated chitinous exoskeleton
    • a waxy covering on the cuticle (protection against desiccation)
    • exoskeleton fortified with CaCO3 in marine species
    • tagmosis (head, thorax, abdomen)
    • an open circulatory system
    • a haemocoel as main body cavity
    • a coelom reduced to a gonocoel and pericardium.

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    Energy Efficient Design

    Thin muscles anchored to the inside of the rigid
    exoskeletal plates create a lever system.

    Very small muscle contractions will move the plates
    (and the animal) quickly, with little energy expenditure.

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    The Compound Eye

    The arthropod compound eye is composed of ommatidia,
    each of which provides a small unit of the entire field of vision.

    The number of ommatidia per eye varies among species.
    The more ommatidia, the more finely resolved the image.

    Dragonflies have more ommatidia/unit area than any other arthropod.

    These aerial predators have evolved high visual acuity
    that allows them to localize and capture small, flying prey in mid-flight.

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(click on pic for source)

    Arthropod Development and Metamorphosis

    Different arthropod species undergo different degrees
    of development and metamorphosis.

    These can be categorized as

    • simple metamorphosis
    • complete metamorphosis
    • mixed metamorphosis

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(click on pic for source)

    Simple Metamorphosis

    Simple metamorphosis
    • is also known as ametabolous or gradual metamorphosis
    • no free-living larval stage.
    • juveniles have the same form as the adult, but smaller.
    • juvenile form is called a nymph

    Examples: silverfish, springtail, grasshopper, cockroach

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(click on pic for source)

    Complete Metamorphosis

    Complete metamorphosis
    • is also known as holometabolous metamorphosis
    • Larval and adult morphologies are distinctly different.
    • Morphology changea in stages.
    • Typically, developmental stages are
      egg --> larva --> pupa --> adult

    Examples: butterflies, moths, flies, beetles

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(click on pic for source: watch the dragonfly emerge!)

    Mixed Metamorphosis

    Mixed metamorphosis
    • is also known as hemimetabolous metamorphosis.
    • Larval form (nymph) is different from adult form.
    • Larval form is reminiscent of adult form.
    • An aquatic nymph is called a naiad
    • Sequential metamorphic changes generate the fully adult form.

    Examples: dragonfly, damselfly, cicada

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Arthropod Diversity

    Trilobites (all extinct)

    These iconic arthropods
    • were common in shallow waters of the Paleozoic oceans (250 mya).
    • had a bilaterally symmetrical, metameric body
    • had a chitinous exoskeleton
    • exhibited early tagmosis of the head and thorax approaching a cephalothorax
    • had paired, undifferentiated appendages on each segment

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    Chelicerata

    The chelicerates are united by both molecular and morphological similarity.

    Chelicerata includes spiders, scorpions, mites, ticks, horseshoe crabs, and more.


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illustration by Alexei Kouprianov

    Chelicerae

    All chelicerates have pinching/stabbing mouthparts
    known as chelicerae.

    Depending on species, these can be one of three basic types:

    • (A) jacknife
    • (B) scissor
    • (C) three-segmented

    ...each adapted for a specific feeding natural history.

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    Chelicerata: Horseshoe Crabs

    • Only four extant species
    • Most common is the Horseshoe Crab, found along our Florida coastlines.
    • Compound eyes are embedded in the flexible carapace.
    • Horseshoe crabs walk along the ocean bottom on their jointed legs.
    • They feed on worms and other small prey.
    • Their blood could save your life. (see video)

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    Chelicerata: Arachnida

    Arachnida includes spiders, scorpions, mites, ticks.

    Nearly half of the 100,000 arachnid species are spiders.
    Take that thought to bed with you.

    The arachnid body is made up of a

    • cephalothorax (fused head and thorax)
    • abdomen
    • eight pairs of walking legs on the thorax
    • one pair of chelicerae
    • one pair of pedipalps (for sensing or feeding)

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    Myriapoda
    Centipedes and Millipedes

    These terrestrial arthropods are named for their many walking legs.

    Chilopoda - The Centipedes
    • two (one pair) walking legs per segment
    • body is dorsoventrally flattened in cross-section
    • terrestrial carnivores
    • scary as %$^*
    Diplopoda - The Millipedes
      • four (two pairs) of walking legs per segment
      • body is round in cross section
      • entirely herbivorous and detritivorous

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    Pancrustacea: Insects

    Insects (Hexapoda) have chewing mouthparts called mandibles.

    There are more than 900,000 described species.
    Beetles alone number more than 350,000 species Body consists of a

    • head
    • thorax
    • abdomen
    • six (three pairs) of thoracic walking legs
    • two to four wings attached to the thorax
    • some derived insect taxa have lost their wings

    A biologist who studies insects is an entomologist

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    Pancrustacea: Crustaceans

    Like insects, crustaceans have mandibles.

    This diverse group includes crabs, shrimp, lobsters, barnacles,
    isopods, amphipods, mantis shrimp
    and a host of others
    to the tune of 67,000 species.

    • The chitinous exoskeleton is fortified with CaCO3
    • Most are marine, but many live in freshwater or are terrestrial.

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