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
a) strongly metemaric body plan, tagmatization widespread & varied
b) semi-rigid external skeleton, often jointed
c) the acron & telson (bears the anus) are considered homologous to the annelid prostomium & pygidium; motile body cilia absent!
d) thoracic segments bear well-developed, jointed limbs
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 the 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
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)
a) limb joints often have alternating ball & socket (di-condylic) hinges in adjacent segments
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Copyright © 1998 by A. Richard Palmer. All rights reserved.(revised March 16, 1998)