Phylum Arthropoda

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Phylum Arthropoda
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• Similarities between Annelids and Arthropods
• Arthropods are metameric and their segments have
  appendages
• Nervous system with ventral nerve cords

• Phylum Onychophora
• Annelid-like
• Segmented unjointed appendages similarity in
  structure of the body wall segmentally arranged
  nephridia pigment-cup ocelli
• Arthropod-like
• Reduced coelom, open circulatory system,
  tracheal system soft cuticle composed of chitin

Peripatus, a small, nocturnal form found among
the leaf-litter of tropical forests of South
America.
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The fossil record indicates that onychophorans
have not changed much in 500 million years.
The Onychophoran Aysheaia from the Cambrian. A
lobe-limbed, segmented animal. Also note the
spines on the legs. The head end has a pair of
tapering limbs with spines, and three small
projections near the mouth. Traces of the
digestive tract can also be seen.
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Phylum Tardigrada or water bears also have
features in common with both annelids and
arthropods

• Annelid-like
• Unjointed (8) legs annelid-type nervous system
• Arthropod-like
• Presence of a cuticle (nonchitinous) that is
  periodically molted similar attachment of muscle
  fibers to exoskeleton

• One of the most interesting features of
  tardigrads is their ability to undergo
  cryptobiosis

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Arthropod Taxonomy Overview The arthropods
evolved along four main lines, which most
zoologists recognize as 4 distinct subphyla 1.
Trilobita - extinct trilobites 2. Chelicerata -
horseshoe crabs, spiders, ticks, mites, and some
extinct groups 3. Crustacea - crabs, lobsters,
shrimps, barnacles 4. Uniramia - insects,
centipedes, millipedes
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• The Arthropod Exoskeleton
• Epidermis secretes an external skeleton called
  the exoskeleton
• Advantages of possessing an exoskeleton
• provides strong support
• provides rigid levers that muscles can attach to
  and pull against
• offers protection
• serves as a barrier to prevent internal tissues
  from drying out important because many
  arthropods live on land
• serves as a barrier to prevent infection

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• Structure of the Exoskeleton
• Composed of the polysaccharide chitin and
  protein - glycoprotein
• Outer surface called the epicuticle contains
  waxes
• The thicker portion is called the procuticle
• exocuticle
• endocuticle
• In the exocuticle, the glycoprotein chains are
  cross linked process is called tanning

epicuticle
exocuticle
procuticle
endocuticle
epidermis
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• Molting
• In order to grow the arthropod must shed its
  exoskeleton, and secrete a new and larger one -
  molting or ecdysis.

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• Jointed Appendages
• Exoskeleton divided into a number of plates and
  cylinders

• At the junction point between plates and
  cylinders, the exoskeleton remains thin and
  flexible these are the joints
• Jointed appendages allows arthropods to move
  efficiently and quickly
• Muscles are integral to arthropod movement they
  attach to the inner side of the exoskeleton they
  often function as a lever system

Arthropod joint
Vertebrate joint
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• Specialized Arthropod Segments Reduction in
  Metamerism
• The evolution of the arthropods witnessed a
  reduction in metamerism
• The arthropods evolved modified groups of
  segments (e.g., segments became lost, some fused
  together
• The fusion of groups of segments into functional
  groups is called tagmatization
• In so doing, various appendages on segments
  became specialized for functions other than
  locomotion, e.g. prey capture, filter feeding,
  sensing various kinds of stimuli, gas exchange,
  copulation, etc.

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• Arthropod Respiratory Advances
• Special respiratory structures allow the
  arthropods to metabolize more efficiently and
  thus move rapidly
• High metabolic rates require rapid oxygen
  delivery, and arthropods can accomplish this with
  respiratory organs that have a large surface area
  for collecting oxygen quickly

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• Gills
• Many aquatic arthropods (crabs and lobsters)
  have gills, which are typically modifications of
  appendages or outgrowths of the body wall - folds
  of tissue with a large surface area

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• Tracheae
• Gas exchange organs among terrestrial arthropods
  is usually internal invaginations of the
  integument

• Insects have tracheae, branching networks of
  hollow air conducting tubes such that air is sent
  to every cell in every tissue

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• Book Lungs
• Spiders have book lungs, chambers with leaf-like
  plates for exchanging gases air flows over the
  plates and blood flows through them

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• Acute Senses
• Arthropods have a well-developed nervous system
  that is of the same overall design as the
  annelids anterior brain and a double, ventral
  hollow nerve cord.
• The sensory receptors of arthropods are usually
  associated with modifications of the chitinous
  exoskeleton
• The head usually bears various kinds of sense
  organs (e.g. antennae) with extreme sensitivity

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• Acute Senses cont.
• Many arthropods have compound eyes - eyes that
  are composed of many visual units called facets
  (ommatidia) capable of color vision and
  detecting the slightest movements of prey or
  predators
• Some eyes are simple eyes with only a few
  photoreceptors however, they are capable of
  forming crude images

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• Digestive System
• Divided into 3 main regions foregut, midgut, and
  hindgut
• Foregut and the hindgut are lined with chitin
• Foregut is involved with ingestion, mechanical
  breakdown, and storage
• Hindgut is involved with water absorption and
  formation of the feces
• Midgut is not lined with chitin involved with
  digestion and absorption
• Outpockets (e.g. digestive glands) increase the
  surface area for digestion and absorption

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• Internal Transport and Excretion
• Open circulatory system
• Many crustaceans possess an excretory organ
  called the green gland (antennal gland), which
  filters fluid from the blood

• Most insects and spiders have a excretory system
  called malpighian tubules

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• Reproduction
• Sexes are separate fertilization is external in
  aquatic forms, internal among the terrestrial
  forms

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Arthropod Diversity
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Subphylum Trilobita

• A group of extinct marine arthropods.
• Ranged in size from a few millimeters up to 75
  centimeters.
• Body divided into three parts
• cephalon (head), a single plate made up of
  several fused segments
• thorax, consists of a number of segments hinged
  together
• pygidium (tail), also segmented, but like the
  head, fused together into a single plate.
• Compound eye, composed of radially arranged
  visual units

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• Subphylum Chelicerata
• Lack antennae
• Body is usually composed of two regions
  cephalothorax and abdomen
• Cephalothorax is usually covered dorsally by the
  carapace
• Six pairs of appendages first pair are modified
  feeding structures - chelicerae
• Second pair are called pedipalps

• Four additional pairs of appendages are walking
  legs
• No abdominal appendages
• Some have compound eyes, usually have simple
  eyes capable of forming crude images.

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• Class Meristomata (e.g., Horeshoe crabs)
• Marine chelicerates, common off of the Atlantic
  and Gulf coasts
• Large dorsal carapace bearing compound eyes
• Possess chelicerae, pedipalps, and 4 pair of
  walking legs (all but the last pair are chelate
• Abdomen terminates in a long tail called the
  telson used to turn the animal right side up
• Possess a series of gill plates called book gills

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• Class Arachnida
• Among spiders, the cephalothorax and abdomen
  shows no external segmentation tagma are joined
  by a narrow pedicel
• Respiration is accomplished via book lungs,
  tracheae, or both
• Usually have 8 simple eyes at the very least
  they detect motion for some of the predatory
  forms, they are capable of forming crude images
• Many species have evolved poison glands
  associated with the chelicerae

Black widow
Brown recluse
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• Class Arachnida cont.
• Many of the spiders and mites are capable of
  producing silk produced by silk glands that open
  to the exterior part of the abdomen through
  spinnerets
• Silk is used to build webs for trapping prey,
  nests which serve as retreats, and egg cases it
  is also used to form a dragline
• Most spiders are predaceous and have all kinds
  of sensory hairs and relatively well-developed
  eyes for motion detection

Orb web construction
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• Prey capture among the spiders
• Some species are cursorial predators, those that
  stalk and ambush their prey they usually have
  well-developed eyes

Jumping spider

• Others are web building spiders, those that
  construct various kinds of webs made of silk to
  trap their prey
• Eyes are not as well developed as cursorial
  predators, but they have a battery of sensory
  hairs for detecting vibrations

Grass spider
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• SubPhylum Crustacea
• Possess 2 pair of antennae First pair is
  homologous to those of insects second pair is
  unique to the crustaceans
• Second antennae have various functions,
  including sensory, locomotion or feeding.
• The head bears a pair of compound eyes and 3
  pairs of mouthparts a pair of mandibles, and 2
  pairs of maxillae used for food handling

• Trunk varies considerably among classes
• Primitively, the first three pairs of thoracic
  segments are maxillipeds function in handling
  food
• Also, there are usually 5 pairs of appendages
  strengthened for walking (walking legs) and
  protection (chelipeds, pincer-like claws)

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• SubPhylum Crustacea cont.
• Abdomen is also highly variable, but it is
  primitively large
• Groups with a well-developed abdomen usually
  possess six pairs of appendages Five pairs of
  structures called swimmerets (pleopods) one
  pair of structures called uropods,
• Uropods together with the terminal telson form a
  tail fan than can serve as rudders during
  locomotion

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• SubPhylum Crustacea cont.
• Primitively many of the appendages of the
  crustaceans are biramous there is an outer
  exopod and an inner endopod
• They usually have an extremely hardened
  exoskeleton, which is impregnated with calcium
  carbonate - carapace

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• SubPhylum Crustacea cont.
• The primitive larva of the crustaceans is called
  the nauplius larva
• It has an unsegmented body, a frontal eye, and 3
  pairs of appendages, representing the 2 pairs of
  antennae and the mandibles

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Crustacean Diversity
Ostracod
Water flea
Copepod
Fairy shrimp
Fish louse
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Barnacles are Crustaceans!
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• SubPhylum Uniramia
• Single pair of antennae
• First pair of feeding appendages are mandibles
• There are one or 2 pairs of maxillae
• Number of legs vary from 3 pair to many pairs
  they are unbranched or uniramous

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• SubPhylum Uniramia cont.
• Class Chilopoda (Centipedes)
• Serial segmented, flattened body and each
  segment has a pair of jointed appendages
• Active predators, killing their prey with poison
  claws, which are modified legs on first segment

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• SubPhylum Uniramia cont.
• Class Diplopoda (Millipedes)
• Serially segmented, rounded body with 2 smaller
  pairs of legs per segment
• Slow moving feed on decaying plants

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• Class Insecta
• Body is divided into 3 parts the head, thorax
  and the abdomen.
• Head has one pair of antennae, a pair of
  compound eyes and several sets of simple eyes
• Mouthparts a pair of mandibles, and two pairs
  of maxillae
• One pair of maxillae are fused together to form
  a lower lip - labium
• An upper lip - labrum - formed from an extension
  of the head

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• Class Insecta cont.
• Mouthparts are highly modified depending on the
  group you are discussing
• Mosquitoes have pointed mouthparts for piercing
  and sucking grasshoppers have mouthparts that
  are well adapted for chewing butterflies for
  siphoning flies for sponging

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• Class Insecta cont
• Thorax is composed of 3 segments and each one
  has a pair of legs the last two segments also
  have a pair of wings.
• Wings of insects are modified portions of the
  exoskeleton
• 1st pair is usually tough and leathery and fold
  over the inner pair for protection.
• Abdomen does not have appendages terminal
  portions do harbor the reproductive structures

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• Most insects undergo metamorphosis
• Incomplete Metamorphosis (Hemimetabolous)
• Early developmental stages are very similar to
  the adults only the wings and the reproductive
  structures gradually develop
• The immature stages are called nymphs
• Thus development is egg----gt nymphs ----gt adult

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Complete Metamorphosis (Holometabolous)

• Each of the developmental stages is structurally
  and functionally very different
• The egg develops into an immature larva eats
  voraciously
• Followed by a transitional stage - pupa,
  contained within cocoon
• Metamorphosis occurs within the pupal
  exoskeleton, yielding a sexually mature adult