Arthropods have jointed appendages and an exoskeleton. The exoskeleton has three layers. The lipoprotein outer layer provides water proofing. The chitin middle layer provides hard protection. The flexible inner layer allows movement. Like the annelids, their bodies are segmented.
Members of the class Crustacea are all aquatic (except for the pill bug) and have gills. They have five pairs of legs and two body regions: the cephalothorax and the abdomen. Examples include lobsters, crabs, and barnacles. Members of the class Chilopoda have many body segments with one pair of legs per segment. An example of a chilopod is a centipede. Members of the class Diplopoda also have many body segments but they have two pairs of legs per segment. An example of a diplopod is a millipede. Members of the class Arachnida have no antennae and four pairs of legs. Their first appendages are poisonous pinchers called chelicerae. Examples include spiders, scorpions, and ticks. Members of the class Insecta have three pairs of legs and one or two pairs of wings. Their body is divided into three sections: the head, the thorax, and the abdomen.
Arthropods are protected by fast movement, an exoskeleton, pinchers, camouflage, and poison glands.
The fast movements of arthropods are the combined result of jointed appendages, a hard skeleton, and fast acting striated muscle.
Feeding & Digestion
Arthropods have very varied diets. Digestion is extracellular in the gut and there is a division of labor.
Arthropods have an open circulatory system. They have a dorsal heart and arteries.
Crustaceans excrete through nephridia or green glands. Others use a network of Malpigian tubes which collects liquid wastes in the gut. Wastes are crystallized and then excreted to reduce weight for flight.
Crustaceans use gills. Insecta, chilopoda, and diplopoda use trachea and spiricles. Arachnids use book lungs.
Arthropods have a highly complex nervous system with a brain and a ventral nerve cord. They also have compound eyes, proprioceptors, touch receptors, chemoreceptors, and auditory receptors.
Most have separate sexes and internal fertilization. Most undergo some form of metamorphosis.