The arthropods are the most successful type of organism known so far - if you define success by the number of known species. Almost half of the described species are arthropods, and given the rate at which scientists find new species, we can presume that we have described less than one fifth of the species out there. They are bilaterally symmetrical (there is a left/right symmetry), their bodies are made up from a series of segments, and they have paired and usually jointed appendages on some or all of the body segments. The body is protected by a tough organic or organic-mineral cuticle which functions as an exoskeleton. In order to grow, arthropods periodically shed their cuticle by a process called ecdysis. Insects, spiders, crustacea, trilobites are all types of arthropods.