Zoology 250 (2012) ARTHROPODA (I): General features ( Arthropoda study images; click on tree to see full cladogram)

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1) The Arthropoda is the largest animal phylum (more than 1,100,000 spp)

a) strongly metameric body plan, tagmatization widespread and varied

b) the acron and telson (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 entire exoskeleton (cuticle) must be molted

a) two main functions: protection and support

b) each body segment has 2 or 4 plates (sclerites): dorsal tergite (=notum), two lateral pleurites (Crustacea and Uniramia only), ventral sternite

c) the cuticle has 3 main layers; all 3 are thin thin and flexible at joints:

- epicuticle: protein only; thin, may be waxy in insects; molted
- exocuticle: chitin, proteins (may be tanned) and minerals; molted
- endocuticle: chitin, proteins and minerals; resorbed before molting

d) molting 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 planes vary among subphyla

3) Jointed limbs exhibit a tremendous diversity of form:

a) crustacean limbs have 3 components (protopod, exites, endites)

b) the terminal exite= exopod, the terminal endite= endopod

c) non-terminal branches include epipod (an exite) and gnathobases or oostegites in peracarid crustaceans (both are 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 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