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Joint-Legged Animals 465/8. By: Amina Nur. Animals With Exoskeletons (Phylum Arthropoda ). In terms of numbers, Arthropods dominate animal life 9 000 000 arthropods are estimated to be undiscovered All have segmented bodies, suggesting evolution from segmented Annelid Worms

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animals with exoskeletons phylum arthropoda
Animals With Exoskeletons(Phylum Arthropoda)
  • In terms of numbers, Arthropods dominate animal life
  • 9 000 000 arthropods are estimated to be undiscovered
  • All have segmented bodies, suggesting evolution from segmented Annelid Worms
  • Segments become more specialized in higher Arthropods, nearly all segments differ in function
  • They developed exoskeletons, joint legs and a hemocoel(blood cavity)
  • Exoskeleton is a thick external covering composed of strong waterproof carbohydrate molecules called Chitin
  • Protects animal, resists desiccation (drying out) in non aquatic environments, allowing them on land
  • Rigidity prevents efficient locomotion
  • Evolved appendages and groups of muscles which evolved into joints increasing mobility
  • No longer needed coelom to provide body shape
  • Reduced ability for gas exchanged. Aquatic arthropods developed gills that took in O2 and expelled CO2
  • Many arthropods evolved Tracheae to carry O2 to body
  • Rigidity hindered growth, they shed exoskeleton when they grew and replaced them with a bigger one (adapted by moulting)
  • Circulatory system has a hemocoel
  • Over time their bodies replaced body fluid with bloodwhich travelled through vessels and emitted into hemocoel
  • It bathed organs directly in an arrangement called Open Circulatory system
  • Developed sensory receptors: eyes and antennae

Segments of exoskeleton modified in different arthropod groups. Specialized regions, modified, joint appendages, and groups of muscles improved arthropod function and mobility.

class arachnida
Class Arachnida
  • Includes: Scorpions, spiders, mites and ticks
  • Body has 2 parts: head fusing to body segments forming cephalothorax
  • Posterior body segments (abdomen) contain most organs
  • Arachnids have 6 pairs of appendages – each for different functions:
    • To feed
    • Sense environment
    • The last 4 are used to walk


  • Male inserts sperm into small sac in the female called the seminal receptacle where fertilization occurs

Life cycle:

  • Miniature adults hatch from eggs and live independently from the time they hatch
  • Gas exchanges through book lungs (highly folded membranes) in abdomen
  • Silk glands in spiders’ abdomen produce protein threads for spinning webs
  • Many spiders build webs with their silk, but some use their long threads to mate or protect young
class crustacea
Class Crustacea
  • 40, 000 species
  • 3 Regions make up body:
    • Head
    • Thorax
    • Abdomen
  • Exoskeleton forms thick carapace (covers head + other segments)
  • 2 appendages, antennae, followed by the mandibles
  • Mandibles crush food and feed it to the mouth
  • Behind mandibles, a pair of millipedes – sense environment, search for food
  • Behind that, large claw like Chelipeds grab food and protect themselves from predators
  • Four legs follow the Chelipeds
  • On the abdomen, 6 pairs of swimmerets help crustacean swim
  • Pair of flat appendages (Uropod) lie on tail (Telson) behind posterior end of abdomen
  • These act as paddles, moving the animal backwards
  • They have many gills, connected to walking legs, which is protected by Carapace
  • Gils are feathered to increase gas exchange surface area
  • Gases in water diffuse into and out of the gills and travel through the circulatory system to body tissues
  • To increase diffusion through blood vessels of gills, swimmerets on telson beat and sweep water forward, under carapace and over the gills.
  • Males and females come together to copulate
class insecta
Class Insecta
  • Numerous of all animals species
  • + 1.5 million species
  • Insect bodies are like those of a crustacean
  • On their heads one antenna senses the environment
  • On the thorax, there are 3 pairs of legs
  • Gas exchanged carried out through air sacs and tubes (trachea)
  • Adults of flying insects, usually have two pairs of stiff membranous wings attached to the thorax
  • Flies have only one. In beetles, front wings form hardened wing covers
  • Separate sexes to produce egg and sperm
  • Some insects undergo simple development, hatching like miniature adults
  • Winged insect undergo complex developments
  • Body structure changes radically in process of metamorphosis – change of shape and diet
  • Transform from worm-like larva maturing through each instar, or early stages into adults
  • Diet change minimizes the competition for food between larvae and adults
  • This increase species’ chances for survival
class diplopods and chilopods
Class Diplopods and Chilopods
  • Diploids  millipedes
  • Millipede suggest the insect has thousands of feet
  • Members of this class only have about a hundred pair of legs
  • Number of segments in the body range from 10 – 100
  • Each segment has 2 pairs of feet (Diplopod means ‘double footed’)
  • Chilopods are known as centipedes
  • Like Diplopods, Chilopods only have about 10 – 100 pairs of legs
  • Each segment contains only one pair
  • Chilopods:
  • Diplopods:
joint legged animals

Joint-Legged Animals

By: Amina Nur 

Biology sucks…we all know nobody was really listening…