1) The Arthropoda is the largest animal phylum (more than 750,000 spp)
a) strongly metameric body plan, tagmatization widespread and varied
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
2) The semi-rigid, jointed exoskeleton (cuticle) must be molted
a) a body segment may have up to 4 plates (sclerites): dorsal tergum (=notum), two lateral pleura (Mandibulata only), ventral sternum b) the cuticle has 3 main layers (epicuticle, exocuticle, endocuticle) formed mostly of chitin and protein, but may include wax, tanned proteins and minerals (e.g., CaCO3); it is thin and flexible at joints
c) molting is controlled by ecdysone; only epi- and exocuticle is shed (includes lining of foregut, hindgut, trachea, gills, and apodemes)
d) new cuticle is produced underneath the old one before it is shed
e) following a molt, water or air inflates the body to its new size before the cuticle hardens again
f) molting fracture planes vary among subphyla
3) Jointed limbs exhibit a tremendous diversity of form:
a) crustacean limbs have 3 components (protopodite, exite, endite)
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