Lecture 23: Onychophora, Arthropoda (I)

Onychophora

• Ecdysozoa → panarthropoda → onychophora

• velvet worms

  • named for their soft cuticle which repels water

Onychophora Body Plan

• special type of segmentation

• 13-43 pairs of unjointed legs with pads and 2 claws

• muscles in isolated bands, forming the body wall as a sheet-like muscle layer

• jaws likely homologous to chelicerates

• slime papillae

  • shoot slime to entrap prey

Onychophora Circulatory System

• mixocoel/hemocoel body cavity in adults with unique hemal channels

Onychophora Respiration

• gas exchange by tracheae and spiracles

  • probably not homologous with arthropods

Onychophora Reproduction

• gonochoristic

  • 1 species parthenogenic

• oviparous, viviparous (most), ovoviviparous

Onychophora Mass Extinctions

• onychophores have survived all 5 mass extinctions, how ?

  • FIND ANSWER

Arthropoda Body Plan

• body internally and externally segmented

  • divided into head (cephalon) and trunk, and commonly with further regional body specialisation (tagmosis)

  • each body segment with a pair of jointed appendages

• cuticle forming a well-developed exoskeleton with sclerites

• coelom reduced to portions of the reproductive and excretory systems

• muscles metamerically arranged, striated and grouped in bands; dorsal and ventral longitudinal muscles present (no circular musculature)

Arthropoda Head

• has a labrum

  • ring of fibrous cartilage, stabilize the ball-and-socket joint

• typically has a head shield or carapace

• pair of compound eyes and one-several simple median ocelli

  • ocelli lost in several groups

Arthropod Circulatory System

• largely open, main body cavity an open hemocoel

  • mixocoel

• dorsal heart muscular pump with lateral ostia for blood return

Arthropoda Digestion

• gut complex and highly regionalized

Arthropoda Nervous System

• with dorsal ganglia (cerebral ganglia) and ganglionated ventral nerve cords

Arthropoda Reproduction

• most gonochoristic; direct, indirect, or mixed development

  • some parthenogenetic

Trilobita

• Phylum Arthropoda → Class Trilobita

• extinct group of arthropods

• body composed of cephalon, thorax, and pygidium

  • body divided into 3 longitudinal lobes, running from head to tail

• 1 pair of antennae

• exclusively marine

Trilobita Extinction

• last trilobites went extinct around 245 MYA

  • end Permian mass extinction, though started to decline in Ordovician/Devonian

• well represented by the fossil record

Trilobita Legacy

• helped with the theories of continental drift

• estimate speciation rates of Cambrian Explosion

• most diverse group of metazoans in the fossil record

• good stratigraphic markers

  • the geological study of rock layers (strata) and layering (stratification), focusing on the distribution, deposition, and age of sedimentary, igneous, and metamorphic rocks