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

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