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-like: growth by moulting; reduced coelom; hemocoel & heart; gut lined with cuticle; trachae/spiracle system; large brain
c) unique traits: no external segmentation, lobopods, slime papillae
a) divided into four subphyla: Chelicerata (spiders, mites & relatives), Trilobitomorpha (extinct trilobites), Crustacea (crabs, lobsters & relatives), Uniramia (insects & relatives)
b) characters distinguishing subphyla are: number of tagmata, location of mouth in embryo & adult, number of adult anterior segments with antennae, anterior limb diversity
c) the acron & telson relate to the annelid prostomium & pygidium
d) thoracic segments bear well-developed limbs to walk or swim
a) each segment bears four plates: tergum, 2 lateral pleura, sternum
b) the cuticle is composed of many layers and may include wax, tanned proteins & minerals (e.g., CaCO3); it is flexible at joints
c) molting is controlled by ecdysone; only the exocuticle is shed
d) following a molt, water or air inflates the body to its new size
a) limb joints often have ball & socket (di-condylic) hinges
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Copyright © 1997 by A. Richard Palmer. All rights reserved.(revised March 26, 1997)