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Arthropods. Chapter 18. 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. Phylum Arthropoda. Arthropods are: Multicellular Bilaterally symmetrical Triploblastic

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Chapter 18

phylum arthropoda
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.
phylum arthropoda3
Phylum Arthropoda
  • Arthropods are:
    • Multicellular
    • Bilaterally symmetrical
    • Triploblastic
    • Have a true coelom (protostomes)
    • Segmented
general characteristics of arthropods
General Characteristics of Arthropods
  • The diversity and success of arthropods are largely related to their segmentation, hard exoskeleton (made of chitin), and jointed appendages.
general characteristics of arthropods5
General Characteristics of Arthropods
  • Segments have combined into functional groups called tagmata.
    • Tagmata have specialized purposes.
general characteristics of arthropods6
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.
general characteristics of arthropods7
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.
a versatile exoskeleton
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.
more efficient locomotion
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.
air piped directly to cells
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.
highly developed sensory organs
Highly Developed Sensory Organs
  • Arthropods have a variety of sensory organs.
complex behavior patterns
Complex Behavior Patterns
  • Arthropods show complex behavior patterns.
    • Mostly innate behaviors.
    • Some learned.
  • Intraspecific competition (between members of one species) is reduced because of metamorphosis.
    • Larval forms may be quite different from adults.
subphylum trilobita
Subphylum Trilobita
  • Early arthropods, such as trilobites showed little variation from segment to segment.
subphylum trilobita15
Subphylum Trilobita
  • Trilobites arose during the Cambrian – maybe earlier and lasted for 300 million years.
subphylum trilobita16
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.
    • Pygidium – segments fused into a plate.
subphylum trilobita17
Subphylum Trilobita
  • Most could roll up like pill bugs.
  • Probably benthic scavengers.
  • Many (especially later species) had large, complex, many-faceted eyes.
subphylum chelicerata
Subphylum Chelicerata
  • Chelicerate arthropods include eurypterids, horseshoe crabs, spiders, ticks, mites, scorpions, & sea spiders.
subphylum chelicerata19
Subphylum Chelicerata
  • They have 6 pairs of cephalothoracic appendages:
    • Chelicerae (mouthparts)
    • Pedipalps
    • 4 pairs of walking legs
class merostomata
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.
class merostomata21
Class Merostomata
  • Three genera of horseshoe crabs live today.
  • Limulus, found in North America, has existed on earth almost unchanged since the Triassic period.
class merostomata22
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.
class merostomata23
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.
class pycnogonida
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.
class arachnida
Class Arachnida
  • Class Arachnida includes spiders, scorpions, mites, and ticks.

50 µm

class arachnida26
Class Arachnida
  • Two tagmata:
    • Cephalothorax
      • Chelicerae
      • Pedipalps
      • 4 pairs walking legs
    • Abdomen
class arachnida27
Class Arachnida
  • Most spiders – order Araneae – have 8 simple eyes that can detect light and motion.
    • Some hunting & jumping spiders may form images.
class arachnida28
Class Arachnida
  • Many spin a web used for prey capture.
    • Some chase & catch prey.
class arachnida29
Class Arachnida
  • 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 – females brood young within their reproductive tract.
class arachnida30
Class Arachnida
  • 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.
class arachnida31
Class Arachnida
  • 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.
  • Annelids and arthropods share a number of shared derived characters.
  • Recent molecular evidence has shown them to belong to separate superphyla.
    • Segmentation arose independently.
    • Arthropods did not descend from annelids.