KINGDOM ANIMALIA
| SYSTEMATIC BIOLOGY |
KINGDOM ANIMALIA |
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| PHYLUM ARTHROPODA | |||||
Arthropoda (ar-THRO-po-da) is derived from two Greek roots that mean "jointed foot" [jointed -arthron (άρθρων); and foot -pod (πόδι)]. The reference is to the jointed nature of the feet (and many other structures) on the animals of this phylum.
This is an enormous collection of animals with segmented, chitinous exoskeletons that molt regularly to allow for growth. The primitive arthropods condition is a single pair of legs per segment. Specializations in the forms of the arthropods usually stem from variations in which segments are fused and legs modified or lost. Such specializations have led to body regions (tagmata) with different purposes. In all groups, the anterior segments (usually 5) are fused to form a head with the legs variously modified to form antennae, palps, and mouth parts. They have a reduced coelom and an open circulatory system with a haemocoel. The Phylum Arthropoda is part of a larger natural group known as the panarthropods that includes the Tardigrada, and Onychophora (Brusca and Brusca 2003 and Nielsen 2001) as well as the Pentastoma in this system. I separated the pentastomes from the arthropods because there still is some disagreement as to its relationship with other arthropods groups (Margulis and Schwartz 1998; Tudge 2000; Brusca and Brusca 2003; Nielsen 2001).
Relative Numbers of Living Arthropod Species by subphylum
| Wilmer (1990) gives a list of shared features of arthropods in her "case for monophyly" within the arthropods. I have evaluated these shared features as to whether they are synapomorphic (shared derived) or symplesiomorphic (shared primitive, indicated in red) characters. The symplesiomorphies are shared with the other Panarthropds and as well as other protostomes like the Nematoda and Annelida. | |
| Cuticle, secreted by epidermis, with chitin and protein predominating. | symplesiomorphic characters |
| Localized sclerotization of the cuticle. | symplesiomorphic characters? |
| Metameric segmentation of the trunk with articular membranes between sclerites. | symplesiomorphic characters? |
| Pre-oral segments. | symplesiomorphic characters? |
| Tagmosis (fusion of segments in specialization), especially cephalization. | symplesiomorphic characters |
| Periodic molting of cuticle, controlled by ecdysones. | symplesiomorphic characters |
| Segmented and jointed appendages. | symplesiomorphic characters? |
| Similar inter-segmental tendon systems. | synapomorphic characters |
| Dorsal and ventral longitudinal muscle metamerically arranged. | synapomorphic characters |
| All muscle striated. | synapomorphic characters |
| Muscle tonofibrillae penetrate cuticle. | synapomorphic characters |
| Lack of cilia. | symplesiomorphic characters? |
| Brain with )at least) ocular protocerebrum and usually an antennal deuterocerebrum; paired ventral nerve cords. | synapomorphic characters |
| Compound eyes (the compound eyes of trilobites seem to be unique and different from those of the other arthropods). | synapomorphic characters |
| Cuticular lining of the fore- and hindguts. | symplesiomorphic characters |
| Haemocoel as a primary body cavity; characteristic blood distribution and ostiate heart. Coelom restricted and very reduced. | symplesiomorphic characters |
The arthropods have undergone numerous taxonomic revisions over the past decade. The phylum has been broken into two or three separate phyla (e.g. Margulis and Schwartz, 1998; Millmer, 1990). The review of Millmer (1990) reflects the work of Manton (1977) who suggests that the Chelicerates, Crustaceans, and Uniramids (also called the Mandibulata, a group that includes the Myriapods and Hexapods) all arose independently from a proto-platyhelminth group and thus "arthropod" indicates a structural grade. However, the case for polyphyly in the arthropods seems to have weakened very much in light of molecular phylogenies. Tudge (2000), who reviews molecular taxonomic systems shows the phylum with six major clades with the pycnogonids separate from the chelicerates. I follow the system of Brusca and Brusca (2003) in which the arthropods are divided into five subphyla, which is fairly typical of the current taxonomic treatments of the phylum (see the subphyla below). Click on the following subphyla for descriptions and detailed taxonomies.
This page is maintained by Jack R. Holt. Last modified: 03/12/08