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9 Cards in this Set
- Front
- Back
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Phylum Arthropoda
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old (600 my) and diverse group many marine forms
considered monophyletic internal systematic relationships controversial |
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Key characteristics of arthropods
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(1) Bilaterally symmetric
(2) Segmented both internally and externally (3) Tagmosis = combining of formerly separate segments into fused, integrated units (4) Cuticular exoskeleton composed partly to mainly of chitin; arthropods grow only by molting |
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Trilobites
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4000 described species
wholly marine and extinct excellent fossil record benthic most species probably scavengers up to 70 cm in length - most taxa small monophyletic |
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Crustaceans
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30,000+ described spp.
mostly marine & freshwater: shrimp, crabs, barnacles, isopods, copepods, amphipods a few terrestrial groups (isopods: pillbugs, sowbugs) diverse ecologically and morphologically monophyletic |
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Chelicerates
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65,000+ described species
mostly terrestrial: spiders, mites, ticks, scorpions one extant marine group: horseshoe crabs two tagmata: cephalothorax + abdomen |
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Pycnogonids
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(sea spiders)
4000 described species; all marine Placement with respect to other arthropod groups unclear - probably basal with respect to chelicerates Predatory; some consume algae |
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Myriapods
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13,000 described species - all terrestrial
(A) symphylans, (B) pauropods, (C) centipedes & (D) millipedes monophyly suspect but all have serially repeated post-cephalic segments |
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Hexapoda:
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Entognatha & Insecta > 1,000,000 described species; actual diversity likely much greater Unique form of tagmosis = head + thorax + abdomen Diverse and abundant in terrestrial and many freshwater environments Limited colonization of marine environments
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Synapomorphies of the Hexapoda
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(1) unique form of tagmosis: hexapod tagmata = head + thorax + abdomen (2) Head with six fused segments: three devoted to mouthparts (3) Each of three thoracic segments bears one pair of legs (4) Each thoracic leg with a maximum of six segments in extant forms (5) Small and weak abdominal limbs (if present)
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