Phylum Arthropoda (Crabs, Insects, Shrimp & Spiders)


Class Arachnida Spiders, Ticks & Mites	Class Branchiopoda Fairy Shrimp & Water Fleas
Class Branchiura Branchiurians	Class Cephalocarida Cephalocarids
Class Chilopoda Centipedes	Class Cirripedia Barnacles
Class Diplopoda Millipedes	Class Hexanauplia Hexanauplia
Class Insecta Insects	Class Malacostraca Crabs, Krill & Shrimp
Class Maxillipoda Copepods	Class Merostomata Horseshoe Crabs
Class Ostracoda Ostracods	Class Pauropoda Pauropods
Class Pycnogonida Sea Spiders	Class Remipedia Primitive Crustaceans
Class Symphyla Pseudocentipedes & Symphylans

Members of the phylum Arthropoda are distinguished by having segmented bodies and segmented appendages. The bodies of most arthropods are made of two or three distinct parts-a cephalum (head), a thorax (chest), and an abdomen. They have a hardened external skeleton, often called the exoskeleton, made of the nitrogen-rich polysaccharide chitin. As the animal outgrows the exoskeleton, it is shed and a new one is regenerated. Many arthropods metamorphose: the egg develops into a larval form that differs considerably from the sexually mature adult. Although arthropods range in size from 100 �m to 60 cm long, most adults are about 1 mm long.