Trilobites, Chelicerates, and Myriapods

1
Trilobites, Chelicerates, and Myriapods

• Chapter 19

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Phylum Arthropoda

• Two out of every three known species of animals
  are arthropods.
• Members of the phylum Arthropoda are found in
  nearly all habitats of the biosphere.

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Phylum Arthropoda

• Arthropods are
• Multicellular
• Bilaterally symmetrical
• Triploblastic
• Have a true coelom (protostomes)
• Segmented

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General Characteristics of Arthropods

• The diversity and success of arthropods are
  largely related to their segmentation, hard
  exoskeleton (made of chitin), and jointed
  appendages.

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General Characteristics of Arthropods

• Segments have combined into functional groups
  called tagmata.
• Tagmata have specialized purposes.

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General Characteristics of Arthropods

• As arthropods evolved, the segments fused, and
  the appendages became more specialized.
• The appendages of some living arthropods are
  modified for many different functions.

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General Characteristics of Arthropods

• Arthropods have an open circulatory system in
  which fluid called hemolymph is circulated into
  the spaces surrounding the tissues and organs.
• A variety of organs specialized for gas exchange
  have evolved in arthropods.

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A Versatile Exoskeleton

• The exoskeleton of arthropods is very protective,
  but still flexible.
• The exoskeleton is made of chitin.
• Prevents desiccation.

• Provides places for muscle attachment.
• Does not allow for growth, the outer covering
  must be molted ecdysis.

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More Efficient Locomotion

• Usually, each segment bears a pair of jointed
  appendages.
• The appendages have sensory hairs and may be
  modified for sensory functions, food handling, or
  walking swimming.

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Air Piped Directly to Cells

• Most terrestrial arthropods have an efficient
  tracheal system of air tubes, which delivers
  oxygen directly to the tissues and cells.
• Limits body size.
• Aquatic arthropods breathe using internal or
  external gills.

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Highly Developed Sensory Organs

• Arthropods have a variety of sensory organs.

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Complex Behavior Patterns

• Arthropods show complex behavior patterns.
• Mostly innate behaviors.
• Some learned.

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Metamorphosis

• Intraspecific competition (between members of one
  species) is reduced because of metamorphosis.
• Larval forms may be quite different from adults.

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Relationships Among Arthropod Subgroups

• Clade Panarthropoda, Phylum Arthropoda
• Divided into subphyla based on relationships
  between subgroups based on molecular data.

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Relationships Among Arthropod Subgroups

• Centipedes, millipedes, pauropods, and symphylans
  are placed in subphylum Myriapoda.
• Insects are placed in subphylum Hexapoda.

• Spiders, ticks, horseshoe crabs and their
  relatives form subphylum Chelicerata.
• Lobsters, crabs, barnacles, and others form
  subphylum Crustacea.

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Relationships Among Arthropod Subgroups

• Formerly, insects and myriapods were placed
  together in uniramia.
• Uniramous appendages.
• The mandibulate hypothesis suggests all
  arthropods with mandibles are more closely
  related to each other than to arthropods with
  chelicerae.

• Mandibles in each group may or may not be
  homologous.
• mtDNA data support this hypothesis.

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Subphylum Trilobita

• Early arthropods, such as trilobites showed
  little variation from segment to segment.

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Subphylum Trilobita

• Trilobites arose during the Cambrian maybe
  earlier and lasted for 300 million years.

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Subphylum Trilobita

• Trilobites had a trilobed shape.
• Three tagmata
• Head (cephalon) with a mouth, compound eyes,
  antennae, and 4 pairs of leglike appendages.
• Trunk with a variable number of segments each
  with a pair of biramous appendages.
• One of the branches of biramous appendage was
  fringed and may have been a gill.
• Pygidium segments fused into a plate.

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Subphylum Trilobita

• Most could roll up like pill bugs.
• Probably benthic scavengers.
• Many (especially later species) had large,
  complex, many-faceted eyes.

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Subphylum Chelicerata

• Chelicerate arthropods include eurypterids,
  horseshoe crabs, spiders, ticks, mites,
  scorpions, sea spiders.

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Subphylum Chelicerata

• They have 6 pairs of cephalothoracic appendages
• Chelicerae (mouthparts)
• Pedipalps
• 4 pairs of walking legs
• Lack mandibles and antennae.

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Class Merostomata

• Class Merostomata includes the eurypterids and
  horseshoe crabs.
• Eurypterids were giant water scorpions up to 3 m
  in length.
• Cambrian through Permian.
• Predators, some with large crushing claws.

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Class Merostomata

• Three genera of horseshoe crabs live today.
• Limulus, found in North America, has existed on
  earth almost unchanged since the Triassic period.

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Class Merostomata

• Horseshoe crabs have an unsegmented carapace
  (hard dorsal shield), a broad abdomen, and a long
  telson (tail piece).
• Cephalothorax
• Chelicerae
• Pedipalps
• 4 pairs walking legs
• Abdomen
• 6 pairs of thin appendages
• Book gills found on 5.

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Class Merostomata

• Horseshoe crabs have simple and compound eyes.
• Feed at night on worms and small molluscs.
• Come to shore in large numbers to mate at high
  tide.
• Trilobite larvae resemble trilobites.

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Class Pycnogonida

• Sea spiders, class Pycnogonida, have small, thin
  bodies and usually 4 pairs of walking legs.
• Found in all oceans, most common in polar seas.
• Some have chelicerae and pedipalps.

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Class Arachnida

• Class Arachnida includes spiders, scorpions,
  mites, and ticks.

50 µm
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Class Arachnida

• Two tagmata
• Cephalothorax
• Chelicerae
• Pedipalps
• 4 pairs walking legs
• Abdomen

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Class Arachnida - Order Araneae

• Most spiders order Araneae have 8 simple eyes
  that can detect light and motion.
• Some hunting jumping spiders may form images.
• Sensory setae detect air currents, web
  vibrations, and other stimuli.
• Spiders vision usually poor, so awareness of
  environment depends largely on cuticular
  mechanoreceptors such as sensory setae.

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Class Arachnida - Order Araneae

• All are predaceous, mostly on insects.
• Many spin a web used for prey capture.
• Some chase catch prey.
• Injected venom liquefies and digests the tissues
  which is sucked into spiders stomach.

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Class Arachnida - Order Araneae

• Two or three pairs of spinnerets contain
  microscopic tubes that run to silk glands.
• Liquid scleroprotein secretion hardens as it is
  extruded from spinnerets.
• Silk threads are very strong and will stretch
  considerably before breaking.
• Silk is used for orb webs, lining burrows,
  forming egg sacs, and wrapping prey.

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Class Arachnida - Order Araneae

• Breathe by book lungs and/or tracheae.
• Book lungs unique to spiders - parallel air
  pockets extend into blood-filled chamber.
• Air enters chamber through a slit in body wall.
• Tracheae system is less extensive than in
  insects.
• Transports air directly to tissues.
• Tracheal systems of arthropods represent a case
  of evolutionary convergence.

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Class Arachnida - Order Araneae

• In spiders and insects, Malpighian tubules serve
  as excretory structures.
• Potassium, other solutes, and waste are secreted
  into tubules.
• Rectal glands reabsorb the potassium and water,
  leaving wastes and uric acid for excretion.
• Conserves water and allows the organisms to live
  in dry environments.
• Many spiders have coxal glands, modified
  nephridia, at the base of legs.

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Class Arachnida - Order Araneae

• Reproduction - before mating, male stores sperm
  in pedipalps.
• A courtship ritual is often required before the
  female will allow mating.
• Eggs may develop in a cocoon in the web or may be
  carried by female.
• Young hatch in about two weeks and may molt
  before leaving the egg cocoon.

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Class Arachnida - Order Araneae

• Are spiders really dangerous?
• Spiders are allies of humans in our battle with
  insects.
• American tarantulas rarely bite, and bite is not
  dangerous.
• Species of black widow spiders are dangerous.
• Venom is neurotoxic.
• Brown recluse spider has hemolytic venom that
  destroys tissue around the bite.
• Some Australian and South American spiders are
  the most dangerous and aggressive.

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Class Arachnida - Order Scorpiones

• Scorpions order Scorpiones feed on insects
  spiders which they seize with their pedipalps.
• The last segment contains a bulbous base and a
  curved barb that injects venom.
• Scorpions are viviparous or ovoviviparous
  females brood young within their reproductive
  tract.

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Class Arachnida - Order Opiliones

• Harvestmen order Opiliones differ from
  spiders in that the abdomen and cephalothorax are
  broadly joined rather than constricted.
• Only two eyes
• Abdomen shows segmentation
• Long legs end in tiny claws.

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Class Arachnida - Order Acari

• Mites and ticks order Acari have a fused
  cephalothorax abdomen.
• Mites are tiny 1mm or less.
• Some feed on plant juices and can be major pests.
• Several species of ticks carry diseases such as
  Lyme disease.

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Class Arachnida - Order Acari

• House dust mites - free-living and often cause
  allergies.
• Spider mites - one of many important agricultural
  pest mites that suck out plant nutrients.

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Class Arachnida - Order Acari

• Hair follicle mite Demodex - harmless but other
  species cause mange in domestic animals.
• Human itch mite causes intense itching.

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Class Arachnida - Order Acari

• Tick species of Ixodes carry Lyme disease.
• Tick species of Dermacentor transmit Rocky
  Mountain spotted fever.
• Cattle tick transmits Texas cattle fever.

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Subphylum Myriapoda

• Subphylum Myriapoda includes these classes
• Chilopoda (centipedes)
• Diplopoda (millipedes)
• Pauropoda (pauropods)
• Symphyla (symphylans)

• Use trachea to transport air.
• Excretion usually by Malpighian tubules.

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Class Chilopoda

• Centipedes class Chilopoda contain a few or
  many segments each (except the first behind the
  head and the last two) with a pair of jointed
  legs.
• Last pair of legs has a sensory function.

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Class Chilopoda

• Head appendages
• One pair antennae
• One pair mandibles
• One or two pairs of maxillae.
• Dorsoventrally flattened.

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Class Chilopoda

• Sexes are separate.
• Some lay eggs (oviparous).
• Some have live young (viviparous).
• Young like little adults no metamorphosis.

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Class Chilopoda

• Centipedes live in moist environments.
• They are carnivores, feeding on insects worms.
• Prey is killed with poison claws on the first
  segment.

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Class Diplopoda

• Millipedes (Class Diplopoda) have two pairs of
  legs on each segment.
• Head has one pair each of antennae, mandibles,
  maxillae.
• Body is more cylindrical.

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Class Diplopoda

• Millipedes live in dark, moist places under
  rocks or logs.
• Most are herbivores, feeding on decayed plant
  matter or occasionally living plants.
• Slow moving, coil up when disturbed.
• Toxic or repellent fluids secreted when disturbed.

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Class Diplopoda

• After copulation, female lays eggs in a nest and
  guards them.
• Larvae have only one pair of legs per segment.

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Class Pauropoda

• Live in moist soil, leaf litter, decaying
  vegetation, or under bark and debris.
• Least well known of myriapods.
• Soft-bodied, small (2 mm or less).
• Approximately 500 species.
• Head lacks true eyes, has branched antennae, and
  a pair of sense organs.

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Class Pauropoda

• 12 trunks segments bear 9 pairs of legs but none
  on the first or last 2 segments.
• One tergal plate covers two segments.
• Lack tracheae, spiracles, and circulatory system.
• Probably most closely related to diplopods.

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Class Symphyla

• Live in humus, leaf mold, and debris.
• Male Scutigerella places a spermatophore at end
  of a stalk.
• Female stores the sperm in special pouches.
• Removes and smears eggs with sperm before
  attaching them to moss or lichen.
• Young hatch with only 6 or 7 pairs of legs.

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Class Symphyla

• Small (210 mm) with centipede-like bodies.
• Soft-bodied with 14 segments - 12 segments bear
  legs and one bears a pair of spinnerets.
• Antennae are long and unbranched.
• About 160 species are known.
• Eyeless with sensory pits at base of antennae.

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Phylogeny

• Relationships between subphyla are debated.
• Taxon of Pancrustacea, which includes hexapods
  and crustaceans, is well-supported.
• Phylogenies using molecular data rarely support
  grouping Myriapoda with Pancrustacea.
• There is support for placement of Myriapoda as
  the sister taxon for Cheliceratae.

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Phylogeny

• Biologists assume that the ancestral arthropod
  had a segmented body with one pair of legs per
  segment.
• Evolution caused adjacent segments to fuse and to
  make body regions.
• Hox gene studies indicate that the first five
  segments fused to form the head tagma in all four
  extant subphyla.
• In spiders, Hox gene studies indicate that the
  entire prosoma corresponds to the head of other
  arthropods.

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Phylogeny

• Genetic studies have been helpful in
  understanding the evolution of uniramous and
  biramous appendages.
• Molecular evidence repeatedly places hexapods
  with crustaceans even though hexapods have
  uniramous appendages and crustaceans have
  biramous appendages.
• Leads to the question Did uniramous appendage
  development evolve more than once?

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Adaptive Diversification

• In contrast to annelids, arthropods have
  pronounced tagmatization by fusion of somites.
• Those with primitive characters have appendages
  on each somite.
• Derived forms are specialized.
• Modification of exoskeleton and appendages
  allowed variation in feeding and movement.
• Adaptations made possible by cuticular
  exoskeleton and small size fostered high
  diversity.

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Phylogeny

• Subphylum Trilobita
• Subphylum Chelicerata
• Class Merostomata
• Class Pycnogonida
• Class Arachnida
• Subphylum Myriapoda
• Class Diplopoda
• Class Chilopoda
• Class Pauropoda
• Class Symphyla
• Subphylum Crustacea
• Subphylum Hexapoda