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

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

Biramous Uniramous

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

Foregut Midgut Hindgut

Cornea fixed rhodopsin mosaic 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

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

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