DESCRIPTION OF THE PHYLUM ARTHROPODA (LATREILLE 1829)

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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. The name was coined by Latreille (1829).

INTRODUCTION TO THE ARTHROPODA 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 functions. 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, members of the paraphyletic group, Crustacea. Table 1 is based on the review of the Arthropoda by Willmer (1990), who lists the characters by which she defined the phylum. We have evaluated the characters relative to other protostomes. Of the sixteen characters that she defines, only six seem to be clear synapomorphies for the phylum.

TABLE 1. Willmer (1990) gives a list of shared features of arthropods in her “case for monophyly” within the arthropods. We 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 two decades. The phylum has been broken into two or three separate phyla (e.g. Margulis and Schwartz, 1998; Willmer, 1990). The review of Willmer (1990) reflects the work of Manton (1977) who suggests that the Chelicerates, Crustaceans, and Uniramids (Myriapods + Hexapods) all arose independently from a proto-platyhelminth group and thus “arthropod” indicates a structural grade. However, the case for polyphyly in the arthropods weakened very much in light of molecular phylogenies. Figure 1 (from Figure 1 of Sharma et al. 2014) presents the three current competing hypotheses regarding relationships between major arthropod groups in a monophyletic arthropod group, and in all three cases, the topology of the tree depends on the position of the Myriapoda. Myriapods are basal in the clade of all mandible-bearing taxa (mandibulates) in the Mandibulata Hypothesis (Figure 1A). This follows the proposal by Averof and Akam (1995) of a sister relationship between the crustaceans and insects called Pancrustacea (Zrzavy and Stys 1997). Regier et al. (2008) through a robust molecular analyses confirmed that the insects emerged from within the crustaceans. In this theory, all the mandibulates form a monophyletic clade which is sister to the chelicerates. Furthermore, ‘Crustacea’ forms a paraphyletic set of nested clades within which Hexapoda emerges. The Crustacea and Hexapoda are thus grouped together as the Pancrustacea (also called Tetraconata) in which the hexapods should be understood as terrestrial crustaceans. The Atelocerata Hypothesis (Figure 1B) is similar to the Mandibulata Hypothesis but the crustaceans are basal in the mandibulate clade. This follows long-standing views based on morphological relationships in which the biramous leg (the leg is divided into an outer gill and an inner walking leg) is considered to be ancestral to the uniramous leg (the type of leg found in the chelicerates, myriapods, and hexapods, e.g. Brusca and Brusca (2003), Ruppert et al. (2004), Nielsen (2001), Ax (2000), and Wheeler et al. (2001). In this theory, Atelocerata, also called Uniramia, is the clade that includes Myriapoda and Hexapoda. Otherwise, the mandible-bearing clade is sister to the chelicerates. The Myriochelata Hypothesis (also called the Paradoxopoda Hypothesis, Figure 1C) unites Myriapoda and Chelicerata as a clade which is sister to the Pancrustacea (crustacean clades together with the hexapods). The differences between the Myriochelata and the Mandibulata hypotheses may just be a problem with where the tree of such highly divergent and specialized taxa are rooted (Giribet and Edgecombe 2012), but Rota-Stabelli et al. (2010) consider the Myriochelata association to be due to the problem of long branch attraction. Some of the molecular taxonomic studies (e.g. Lavrov et al. 2004, Regier et al. 2004, Giribet et al. 2004, and Mallatt et al. 2003) threatened to throw Arthropod classification into chaos. However, since Regier et al. (2008), there has been consistent support for a topology similar to that of Figure 1A (see Figure 2; e.g. Rota-Stabelli et al. 2010, Regier and Zwick 2011, Oakley et al. 2012, and Misof et al. 2014). Developmental studies (e.g. Sharma et al. 2014) also support the topology of Figure 1A.

FIGURE 1. HYPOTHESES OF ARTHROPOD RELATIONSHIPS. A. The Mandibulate hypothesis in which all mandibulate taxa are monophyletic with the myriapods basal in the clade. B. The Atelocerata hypothesis in which the uniramid mandibulates are sister groups. C. The Myriochelata hypothesis in which myriapods are sisters to the chelicerates. Figure 1 in part from Sharma et al. (2014).

	Ar = Arthropods Am = Arachnomorpha Ch = Chelicerates Ma = Mandibulates Pc = Pancrustacea Ol = Oligostraca Ac = Altocrustacea Mu = Multicrustacea Al = Allotriocarida
FIGURE 2. MAJOR CLADES OF THE ARTHROPODA A. Relationship of the arthropod subphyla (taxa in the shaded box) within the Ecdysozoa. This is an elaboration of the Mandibulata Hypothesis (see Figure 1A). Crustacea forms a paraphyletic group of nested taxa that includes the Hexapoda in a larger group termed Pancrustacea (Regier et al. 2008).

	EVOLUTIONARY INNOVATIONS IN THE STEM ARTHROPODS 1. compound eyes 2. jointed limbs associated with the head 3. jointed, biramous trunk limbs 4. jointed body 5. specialized appendages associated with the head 6. specialized jointed body segments with sternites (ventral) and tergites (dorsal) 7. reduction in endopodite segments
FIGURE 4. CLADES OF STEM GROUP ARTHROPODS. This is Figure 4 from Edgecombe and Legg (2014), which encompasses Panarthropods with a focus on the stem arthropods. The topology of the cladogram is from Legg et al. (2012 & 2013) and the numbers indicate key evolutionary innovations to the crown group, Euarthropoda, which includes trilobites and the major living arthropod groups of Figure 2.

Figure 4 illustrates mostly extinct, mainly Cambrian, taxa and the series of morphological evolutionary innovations leading to the crown arthropods (Edgecombe and Legg 2014). Significantly, the earliest innovation seems to be complex visual systems, represented by the compound eye (1). Such an innovation is useful for effective predation, as well as predator avoidance and may represent one of the earliest steps in the predator-prey arms race. The succeeding innovations are useful in food item capture and manipulation (2 & 7), movement (3 & 7), protection (4 & 6), and sensing (5). Though not illustrated in Figure 4, the Vendiamorpha (Figure 5) may have been protoarthropods of the Lower Cambrian that exhibited cephalization, but lacked the typical appendages of arthropods. The vendiamorphs were segmented with a kind of head shield (interpreted as cephalization) and dorsoventrally flattened. Almost certainly they crept along the bottom or burrowed in the substrate. Interpretations of vendiamorph fossils are quite difficult, though (e.g. Ivantsov 2004) and they may not be part of the arthropod stem taxa. Anomalopoda (e.g. Anomalocaris) were among the animals known as the lobopodians, which seem to have been active swimmers and among the top predators of the early Cambrian seas (Whittington and Briggs 1985). The animals had an obvious head, large eyes, a pair of arm-like anterior extensions that were too robust to be antennae. Presumably, they were appendages associated with capturing prey and passing them to the circular mouth that worked like a nutcracker. The body of the animal was obviously segmented, but, rather than having jointed appendages, they had a pair of parapodia-like appendages per segment. The general understanding is that Anomalocaris swam by undulating the parapodia in a transverse wave, and thereby, moved rapidly through the water to attack benthic prey from above. That Anomalocaris is a stem arthropod is not universally held, however. Budd (2001) considers the Tardigrades to be descendants of anomalocarids and that their small size is a derived character. Megacheirans (i and j in Figure 4) resembled trilobites with large anterior appendages (the name means large hands). Their position in the arthropod tree is uncertain and arguments have been made for their inclusion in the Euarthropoda as sisters to the Chelicerata (Edgecombe et al. 2011, Lamsdell et al. 2013, Stein et al. 2013, Tanaka et al. 2013). Indeed, the anterior appendages do resemble chelicerae. Edgecombe and Legg (2014), Legg (2013), Daley et al. (2009), Kuhl et al. (2009), and Siveter et al. (2014) hold to the view that the megacheirans were paraphyletic sisters to the Euarthropoda.

FIGURE 5. An illustration of Venda rachiata, a member of the Lower Cambrian marine benthic community. It may have been a protoarthropod. Image by APOKRYLTAROS, Wikimedia Commons	FIGURE 6. An illustration of Anamalocaris, one of the top predators of the Burgess Shale era Cambrian seas. Image by Nobu Tamura, Wikimedia Commons

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By Jack R. Holt and Carlos A. Iudica. Last revised: 02/05/2015