Phylum Arthropoda
• “jointed foot”
• Chitonous exoskeleton
• Open circulatory system
• Polyphyletic or monophyletic?–Merostomata, Crustacea, Trilobites –
biramous– Insects, centipedes, millipedes -
uniramous
Segmentation
• Characteristic of Arthropoda
• Both external and internal segmentation
• Some evidence of segmentation lost
• Regional specialization throughout phylum
Annelid-Arthropod Link
• Segmentation– Segments lost– Segments have fused– Divergence of appendages
• Nervous System
• Spiral determinate cleavage
• Pair of appendages on each body segment
Think about ancestral arthropod
Tagmosis
Cephalization
See Figure 16-3
Uniramous
Biramous
Fig 16-1C
Fig 16-1E
Figure 16-1B
• 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)
Saccate Nephridia
Gas Exchange
• Gills (Crustacea, aquatic insects**)
• Book gills (Merostomata)
• Book lungs (many Arachnida)
• Tracheae (all tracheates [includes insects], Onychophora, Arachnids) - convergence
Down to cellular level and chitin
ForegutHindgutMidgut
Cornea fixedrhodopsinmosaic image
Other Sensory Structures
• Mechanoreceptors
• Chemoreceptors
• Equilibrium receptors
Subphylum Trilobitomorpha
• All extinct
• Present 560 through 260 mya
• Marine
• Diverse – occupied many niches
• Most 3 – 10 cm long
See also Fig 17-2A
Subphylum Chelicerata
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
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
Crustacea
Subphylum Crustacea
• Marine, freshwater, with a few terrestrial• Body in 2 - 3 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
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
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