Phylum Arthropoda - Characteristics
1. EXOSKELETON In most arthropods the body is covered with chitonous cuticle that is hardened into an exoskeleton. In crustacea and millipedes, the cuticle is hardened by the addition of calcium; in insects, the cuticle is tanned, chemically bonded with protein. Once the cuticle is hardened it can not increase in size. Some regions of the cuticle remain unhardened to allow flexibility and movement.
2. METAMERISM
Body composed of numerous segments (somites), segmented condition may be concealed. In the primitive Arthropod, the body was thought to be a series of metameres, each, except for the first and last, with a pair of appendages. Metamerism is an example of an important biological trait, that of replication and modification to develop new traits and capabilities.
3. JOINTED APPENDAGES Jointed appendages give arthropods numerous, generalized appendages which were modified into numerous specialized organs for walking, grasping, and eating.
4. DOUBLE VENTRAL NERVE COR Ventral nerve cord with ganglia(swellings) at each segment. Only the brain (most anterior) ganglia is above the digestive system.