Arthropoda II

• Paleo-Arthropod Phylogeny


  Diversity of forms
• Paleozoic arthropod groups leading to trilobites

  • Trilobite Diversity and Beginnings

  
• Most groups extinct by 350 million years ago
• The last by 250 million years (end Permian: major extinction, 50% of animal general)
• End of Permia is one of the major extinction events in the entire history of this planet, 50% of all animal genera went extinct
• Trilobites


  • Basic tripartite body plan (3 tagmata, tagma II and III fused basically)
  • Uniform appendages (apart from antennae on the head)
  • Fairly simple appendages, each with 2 toes (outer toes: food capture and gas exchange and inner toes for walking)
  • Sensory: antennae, compound dyes (larger in groups that swim), lenses made of calcium carbonate
• Major groups of Chelicerata

  • Spiders(arachnida)
  • Horseshoe crabs (xiphosura)
  • Sea spiders(pycnogonida)
• Chelicerata


  • Lack antennae
  • First (anterior most) segment without appendages
  • Second segment bears chelicerae (short claw/jaw looking arms near their mouths)
  • Mouth lacks mandibles (short chewing appendages seen in later groups)
• Xiphosura (Horseshoe Crab)


  • Often called living fossils
  • Limulus still exact
  • Little change from fossil to living species
  • Head and thorax merged into prosoma (comes from trilobites) ⇒ hear tagma 1 and 2 are merged

  
• Modifications of appendages in Xiphosura

  • Cleaning gill flaps ⇒ book gill
  • Grasping food
  • Chewing food (they chew with their elbows)
  • Walking
  • And in males: first pair of legs modified for grasping female during reproduction
• Blood of horseshoe crab is sky blue
• Horseshoe crab blood


  • Instead of hemoglobin, blood has hemocyanin (Cu not Fe)
  • Has very higher number of amoebocytes ⇒ responsible for clotting blood readily
  • Led to horseshoe crab blood being collected non-lethally for medicinal use (bacterial contamination when blood comes in contact with bacteria and would be able to detect it in a very sensitive way)
  • Tests as sensitive as 0.1 part per trillion
  • Jeak Ding and her husband, ~2000: Singaporean researchers who first synthesized active component "factor C" in the lab (initiates clotting response)
  • can be synthesized in the lab and more stable than natural form
• Arachnida


  • Spiders
  • Ticks
  • Spider mites
  • Scorpions
  • Pseudoscorpions
  • Solpugid

  
• Ticks and mites are NOT insects
• Arachnid Anatomy


  • Head and thorax fused (prosoma)
  • 1 pair of chelicera; 1 pair of pedipalps (touching feet) and 4 pairs of walking legs
  • 0-4 (usually 4) pairs of eyes, some don't have eyes
  • Internal book lungs and/or trachea used for gas exchange without well developed circulatory system
  • Branched gut (diverticula) legs have partial stomach or intestine
  • Chelicerae often with fangs + poison
  • Sometimes silk gland(s) with spinnerets
• Arachnid Adaptation to Land

  • Malphigian tubules for efficient excretion and water retention
  • Wastes collected and dumped into digestive system (hindgut)
  • Acidic environment forces urine (liquid) to precipitate to uric acid (solid); water is reabsorbed
• Arachnid "Silk"


  • Silk housed in liquid form in glands, rapidly polymerizes when released
  • Each gland produces silk for different purpose: web construction (food, mating), safety lines for climbing, protective egg sacks (harder outer, soft inner), air-trapping for "diving bell spiders" that go underwater
• Types of silk glands

  • Aggregate gland
  • Flagelliform gland
  • Cylindrical gland
  • Aciniform gland
  • Pyriform gland
  • Minor and major ampullate gland
• Pycnogonida (Sea Spiders)


  • All species marine with thin slender body
  • Thinker, more flexible exoskeleton
  • Long thin legs
  • Bottom or near-bottom dwellers, eat soft=bodied inverts (hydroids, polychaetes, sponges)
  • 1 pair chelicerae, 1 pair of palps
  • 1 pair ovigers which are grooming of legs and trunk; male use them to carry fertilized eggs
  • Usually 4 pair of walking legs
  • No specialized respiratory, excretory system
  • Gas diffuse in and out, waste diffuse in and out too
  • Digestive system and gonads extend far into legs
  • Abdomen is greatly reduced in size