ARTHROPODA (I): General features ( Arthropoda study images;
|
b) the acron and telson (=pygidium; bears the anus) are considered homologous to the annelid prostomium and pygidium
c) lack of motile body cilia throughout phylum supports monophyly
d) all possess a semi-rigid exoskeleton and jointed limbs
b) each body segment has 2 or 4 plates (sclerites): dorsal tergum (=notum), two lateral pleura (Crustacea & Uniramia only), ventral sternum
c) 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
d) molting is controlled by ecdysone; only epi- & exocuticle is shed (includes lining of foregut, hindgut, trachea, gills, & apodemes)
e) new cuticle is produced underneath the old one before it is shed
f) following a molt, water or air inflates the body to its new size before the cuticle hardens again
g) molting fracture planes vary among subphyla
b) the terminal exite= exopodite, the terminal endite= endopodite
c) non-terminal branches include epipodites (an exite) and gnathobases (an endite)
d) may be uniramous (1 terminal branch) or biramous (2 terminal branches); in uniramous limbs the exopodite is lost (e.g., walking legs of crustaceans and insects)
e) limbs may be phyllopodous (flattened, produce only flap-like motions) or stenopodous (tubular, like a typical walking leg; produce a wide variety of motions like robotic arms)
f) movement is controlled by muscles intrinsic to the skeleton that attach at joint margins or to infoldings of the cuticle (apodemes)
g) stenopodous limb joints often have alternating ball and socket (di-condylic) hinges in adjacent segments
Copyright (c) 2005 by A. Richard Palmer. All rights reserved.
(revised Mar. 3, 2005)