Key Characteristics of Arthropods

1. Exoskeleton
   Composed of protein, lipids, chitin, and usually calcium carbonate 
2. Jointed appendages
   One pair of jointed appendages for each segment
3. Bilateral symmetry
   Head, Thorax, and Abdomen; or Cephalothorax and Abdomen
   
4. Complex muscular system
5. Hemocoel for a body cavity
6. Open circulatory system
7. Complex digestive system
8. Respiratory system
   Gills, trachea, or book lungs
9. Complex nervous system
   Dorsal brain; ventral nerve cord
10. Molting (i.e. growth with ecdysis)

Crayfish Dissection

Crayfish structures that you need to know

External:

• 2 pairs of antennae
• Exoskeleton
• Mouth
• Mandibles
• Maxilla
• Cephalothroax
• Carapace
• Compound eyes
• Swimmerets (Pleopods)
• Uropod
• Telson
  

Internal Anatomy

• Gills
• Stomach
• Gonads
• Heart
• Esophagus
• Digestive gland
• Green glands (excretory)
• Ventral ganglia 
• Brain