8.7 Superphylum Ecdysozoa: Arthropods
Mary Ann Clark; Jung Choi; and Matthew Douglas
Learning Objectives
By the end of this section, you will be able to do the following:
- Compare the internal systems and appendage specializations of phylum Arthropoda
- Discuss the environmental importance of arthropods
- Discuss the reasons for arthropod success and abundance
The superphylum Ecdysozoa also includes the phylum Arthropoda, one of the most successful clades of animals on the planet. Arthropods are coelomate organisms characterized by a sturdy chitinous exoskeleton and jointed appendages. There are well over a million arthropod species described, and systematists believe that there are millions of species awaiting proper classification. Like other Ecdysozoa, all arthropods periodically go through the physiological process of molting, followed by ecdysis (the actual shedding of the exoskeleton), as they grow. Arthropods are eucoelomate, protostomic organisms, often with incredibly complicated life cycles.
Phylum Arthropoda
The name “arthropoda” means “jointed feet.” The name aptly describes the invertebrates included in this phylum. Arthropods have probably always dominated the animal kingdom in terms of number of species and likely will continue to do so: An estimated 85 percent of all known species are included in this phylum! In effect, life on Earth could conceivably be called the Age of Arthropods beginning nearly 500 million years ago.
The principal characteristics of all the animals in this phylum are the structural and functional segmentation of the body and the presence of jointed appendages. Arthropods have an exoskeleton made principally of chitin—a waterproof, tough polysaccharide composed of N-acetylglucosamine. Phylum Arthropoda is the most speciose clade in the animal world, and insects form the single largest class within this phylum. For comparison, refer to the approximate numbers of species in the phyla listed below.
| Phylum | # species |
| Ctenophora | 100 |
| Porifera | 5,000 |
| Cnidaria | 11,000 |
| Platyhelminthes | 25,000 |
| Rotifera | 2,000 |
| Nemertea | 1,200 |
| Annelida | 22,000 |
| Mollusca | 112,000 |
| Nematoda | 28,000+ |
| Tardigrades | >1,000 |
| Arthropoda | 1,134,000 |
| Echinodermata | 7,000 |
| Chordata | 100,000 |
Phylum Arthropoda includes animals that have been successful in colonizing terrestrial, aquatic, and aerial habitats. This phylum is further classified into five subphyla: Trilobita (trilobites, all extinct), Chelicerata (horseshoe crabs, spiders, scorpions, ticks, mites, and daddy longlegs or harvestmen), Myriapoda (millipedes, centipedes, and their relatives), Crustacea (crabs, lobsters, crayfish, isopods, barnacles, and some zooplankton), and Hexapoda (insects and their six-legged relatives). Trilobites, an extinct group of arthropods found chiefly in the pre-Cambrian Era (about 500 million years ago), are probably most closely related to the Chelicerata. These are identified based on their fos