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9 Cards in this Set

  • Front
  • Back
Phylum Arthropoda
old (600 my) and diverse group many marine forms
considered monophyletic
internal systematic relationships controversial
Key characteristics of arthropods
(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
4000 described species

wholly marine and extinct

excellent fossil record


most species probably scavengers

up to 70 cm in length - most taxa small

30,000+ described spp.

mostly marine & freshwater: shrimp, crabs, barnacles, isopods, copepods, amphipods

a few terrestrial groups (isopods: pillbugs, sowbugs)

diverse ecologically and morphologically

65,000+ described species

mostly terrestrial: spiders, mites, ticks, scorpions
one extant marine group: horseshoe crabs

two tagmata: cephalothorax + abdomen
(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
13,000 described species - all terrestrial
(A) symphylans, (B) pauropods, (C) centipedes & (D) millipedes
monophyly suspect but all have serially repeated post-cephalic
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
Synapomorphies of the Hexapoda
(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)