• 1) A small terrestrial phylum of (velvet worms, 70 spp.); marine examples known from Cambrian (e.g., Hallucigenia)

• 2) 'Missing link', many similarities exist with Annelida & Arthropoda:

  • a) annelid-like: body wall cuticle (soft, not articulated) & muscles (polychaete-like); simple gut; little cephalization; paired, segmental nephridia; ciliated reproductive tract; unjointed appendages

  • b) arthropod-likee: growth by moulting; reduced coelom; hemocoel & heart; gut lined with cuticle; trachae/spiracle system; large brain

• 3) Unique traits include: cuticle (unsegmented), lobopods, slime papillae

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• 1) The Arthropoda is the largest animal phylum (approx. 750,000 spp)

  • a) strongly metameric body plan, tagmatization widespread & varied

  • b) the acron & telson (bears the anus) are considered homologous to the annelid prostomium & pygidium

  • c) lack of motile body cilia throughout phylum supports monophyly

  • d) all possess a semi-rigid exoskeleton & jointed limbs

• 2) The semi-rigid, jointed exoskeleton (cuticle) must be molted

  • a) a body segment may have up to 4 plates: dorsal tergum (=notum), two lateral pleura (Mandibulata only), ventral sternum

  • b) the cuticle has 3 main layers (epicuticle, exocuticle, endocuticle) formed mostly of chitin & protein, but may include wax, tanned proteins & minerals (e.g., CaCO3); it is thin & flexible at joints

  • c) molting is controlled by ecdysone; only epi- & exocuticle is shed (includes lining of foregut, hindgut, trachaea, gills, & apodemes)

  • d) following a molt, water or air inflates the body to its new size

• 3) Jointed limbs exhibit a tremendous diversity of form

  • a) may be uniramous (1 branch) or biramous (2 branches)

  • b) crustacean limbs have 3 components (protopodite, exite, endite), & may be phyllopodous (flattened) or stenopodous (tubular)

  • c) body movement is done by muscles intrinsic to the skeleton that attach at joint margins or to infoldings of the cuticle (apodemes)

  • d) limb joints often have alternating ball & socket (di-condylic) hinges in adjacent segments

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