Arthropoda & its Characteristics: Arthropods are invertebrate (without a backbone) animals of the phylum Arthropoda that have a segmented body, jointed legs, and a tough outer covering or exoskeleton. They include insects, crustaceans (lobsters, crabs, shrimp, crayfish), millipedes, centipedes, horseshoe crabs, arachnids (spiders, ticks, and mites) and sea spiders. Together, arthropods comprise the largest and most varied group of invertebrates on Earth
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 (any of two or more isomeric compounds exhibiting metamerism), 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 CORD Ventral nerve cord with ganglia(swellings) at each segment. Only the brain (most anterior) ganglia is above the digestive system.
6. BILATERAL SYMMETRY Body can be divided into two equal halves through one plane. Most animal groups that are very active are bilaterally symmetrical. But not all animals have bilateral symmetry, some have radial symmetry, e.g.. jellyfish.