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Jan 169:52 AM
Over 80% of all known animal species
Arthropoda
Echinodermata
Chordata
Mollusca
PlatyhelminthesNematodaPoriferaAnnelida
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Characteristics of Arthropods• Segmented, bilateral body
• Jointed (arthro) appendages (poda)
• Exoskeleton versitile and adaptable to many environments
• Open circulatory system
• Ventral nerve cord
• Respiration gills, spiracles, or book lungs
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Compound Eye• Thousands of individual photoreception units
• Units receive images from multiple angles
• Allows for an ultrawide view angle so movement can be perceived from all sides
• Small lens size limits resolution so image quality is poor
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Metamorphosis• Metamorphosis is defined as a life process in which abrupt and drastic physical changes occur at well defined stages in an organism’s life
• All arthropods undergo some sort of metamorphosis
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Incomplete Metamorphosis
• Three distinct life stagesEggNymph (wingless but otherwise resembles adult)Adult
• Aquatic insect nymphs are called naiads
• Cockroaches, dragonflies, grasshoppers, & true bugs
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Complete Metamorphosis
• Four distinct life stagesEggLarva (usually wormlike and does not resemble the adult)Pupa (dormant stage sealed within a protective shell)Adult
• Beetles, flies, bees/wasps, & butterflies
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Open Circulatory System• Blood is not confined to blood vessels
• Blood is oxygenated via gills, book lungs, or spiracles
• Heart circulates blood like a prop from a boat in a fish tank
• Body cells directly bathed in blood
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Molting
• Arthropods must shed their exoskeleton in order to grow
• When they emerge from the old shell, they are soft and must grow quickly before the new exoskeleton hardens in a few hours
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Arthropod Subphyla• Many different classifications are used for arthropods. The following is the 5Subphyla model
• Subphylum Trilobita• Subphylum Chelicerata• Subphylum Myriapoda• Subphylum Crustacea• Subphylum Hexapoda
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Subphylum Trilobita• Extinct marine arthropods known collectively as trilobites
• Appeared 540 mya and died out during the PermianTriassic extinction event 250 mya
• 96% of marine species and 70% of land species died out
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Subphylum Trilobita
• Trilobites exhibited lifestyles similar to a wide variety of modern arthropods
• Benthic & pelagic• Filter feeders, scavengers, & predators
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Subphylum Trilobita
• Trilobite Body Plan
• Distinct, relatively large cephalon
• Thorax comprised of transverse segments
• Posterior segments fused to form a tail shield known as a pygidium
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Subphylum Trilobita• The horseshoe crab may be the closest living relative of trilobites
• Trilobite fossils can be found on every continent
• Research using trilobites as a reference led to modern plate tectonic theory