Class Arachnida

• Composed of the spiders, scorpions, ticks, mites and allies; Largest and most important chelicerates
• Nervous organs

  sensory hairs (trichobothria), eyes, and slit senses
• Body segments

  cephalothorax and abdomen

  ○ "Head” portion has sensory hairs and eyes (compound and simple)

  ○ 4 appendages attached to cephalothorax; the two anterior pairs for feeding and the rest for movement.

  ○ Abdomenhas minute pore openings (spiracles), leads to internal lamellae called book lungs
• Circulation

  heart → dorsal anterior & posterior aorta → small arteries → tissue spaces/sinuses → large ventral sinus → book lungs → venous channels → pericardium → ostia → heart
• Excretion

  coxal glands- thin-walled spherical sacs along the sides of the prosoma that collect wastes from the surrounding blood
• Reproductive

  Dioecious, has single or paired gonads in the abdomen, sperm transfer is indirect and through spermatophores, and ducts exit through the genital orifice.
• Nervous System

  • Protocerebrum- contains the optic centers and optic nerves
  • Tritocerebrum- contains the nerves supplying the chelicerae
  • A collar or ring surrounding the esophagus
  • Ganglia has fused with the subesophageal ganglion