ARTHROPODA (I): General features
( Arthropoda study images;
1) Largest animal phylum: >1,250,000 sp., >75% of animal species
b) the acron and telson (bears the anus) believed to be homologous to prostomium and pygidium of annelids
c) lack of motile body cilia throughout phylum supports monophyly!
d) all possess a jointed, semi-rigid exoskeleton & jointed limbs
2) The entire exoskeleton (cuticle) must be molted to grow
b) each body segment has 2 or 4 plates (sclerites): dorsal tergite (=notum), ventral sternite, two lateral pleurites (Crustacea and 'Uniramia' only)
c) the cuticle has 3 main layers; all 3 are thin thin and flexible at joints:
d) molting (=ecdysis) is controlled by ecdysone (hormone); only epi- and exocuticle is shed (includes lining of foregut, hindgut, trachea, gills, and apodemes); endocuticle and 30 - 60% of muscle mass is resorbed!
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) principle molting fracture sutures vary among subphyla
3) Jointed limbs exhibit a tremendous diversity and versatility of form:
b) the terminal exite= exopod, the terminal endite= endopod
c) non-terminal branches include epipod (an exite), and gnathobases (horseshoe crabs) or oostegites (peracarid shrimp) (both= endites)
d) may be uniramous(1 terminal branch= endopod; the exopod is lost or absent) or biramous (2 terminal branches: exopod and endopod)
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 a robotic arm)
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) 2017 by A. Richard Palmer. All rights reserved.
(revised Dec. 19, 2016)