1. Chitonous exoskeleton Open circulatory system
Phylum Arthropoda
“jointed foot”
Chitonous exoskeleton
Open circulatory system
Polyphyletic or monophyletic?
Merostomata, Crustacea, Trilobites – biramous
Insects, centipedes, millipedes - uniramous

2. Segmentation Characteristic of Arthropoda
Both external and internal segmentation
Some evidence of segmentation lost
Regional specialization throughout phylum

3. Annelid-Arthropod Link
Segmentation
Segments lost
Segments have fused
Divergence of appendages
Nervous System
Spiral determinate cleavage
Pair of appendages on each body segment

4. Think about ancestral arthropod

5. Tagmosis

6. Cephalization
See Figure 16-3

7. Biramous
Uniramous

8. Fig 16-1C

10. Fig 16-1E

11. Figure 16-1B

14. Coelom and Blood-Vascular System
Coelom reduced extensively (hydrostatic to rigid skeleton)
Coelom larger in embryonic development
Remnants persist with nephridia and reproductive organs
Circulatory system for transport
Combination of hemocoel and blood vessels (not present in all arthropods)

15. Saccate Nephridia

16. Gas Exchange
Gills (Crustacea, aquatic insects**)
Book gills (Merostomata)
Book lungs (many Arachnida)
Tracheae (all tracheates [includes insects], Onychophora, Arachnids) - convergence

17. Down to cellular level and chitin

18. Foregut
Midgut
Hindgut

21. Cornea fixed
rhodopsin
mosaic image

22. Other Sensory Structures
Mechanoreceptors
Chemoreceptors
Equilibrium receptors

23. Subphylum Trilobitomorpha
All extinct
Present 560 through 260 mya
Marine
Diverse – occupied many niches
Most 3 – 10 cm long

25. See also Fig 17-2A

26. Subphylum Chelicerata

27. Subphylum Chelicerata
Body of two tagmata (regions)
Cephalothorax (prosoma)
abdomen
Cephalothorax – acron + 7 segments
Six pair appendages
1st appendage – chelicerae
2nd pair – pedipalps (often sensory)
Rest are walking legs

28. Subphylum Chelicerata
Abdomen
Primitive condition
Preabdomen (7 segments)
Postabdomen (5 segments and telson)
Varying levels of fusion of abdomen
Many terrestrial, some marine and freshwater

32. Crustacea

33. Subphylum Crustacea Marine, freshwater, with a few terrestrial
Body in tagmata
Head = acron + five segments (fused)
Typical is five pair of appendages (1st and 2nd antennae, 3 pr mouthparts)
2nd antennae homologous to chelicerae
Additional fusion of thoracic segments to head in higher Crustacea

34. Abdomen also variable in number of segments
Subphylum Crustacea
Thorax variable
Depends on if been additional fusion
Cephalothorax characteristic of major crustacean groups
Abdomen also variable in number of segments
Abdomenal appendages - pleiopods

40. Class Hexapoda Uniramous appendages (sP Uniramia)
Most successful group of metazoa
Terrestrial, also freshwater and marine
Three tagmata (H, T, A)
Head – 3 to 7 segments, probably 7
Thorax – 3 segments (pro-, meso-, meta-)
Abdomen – 9 to 11 segments, no appendages