There are over 1,000,000 described species of arthropod; of course only a fraction are parasites. Probably the greatest morphological diversity occurs among the crustaceans. Some species look nearly identical to their free-living relatives except for some minor adaptations for attachment; others are barely recognizable as arthropods. Two areas where morphological divergence is most noticeable are in the mouthparts, which may be modified for piercing the skin of the host, and in the various structures used to attach to the host.