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- 1) 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) & 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 & 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 & socket (di-condylic) hinges in adjacent segments
- 2) Most arthropods have paired, compound eyes (the most spectacular of animal eyes except for vertebrates & cephalopods):
a) composed of multiple ommatidia (units with separate cuticular lenses, light-sensitive cells & nerves)
b) the image is a composite of many spots of light
- 3) Respiration can only take place across very thin cuticle; respiratory adaptations depend upon the type of environment
a) aquatic (depend on some method for circulating water):
i) book gills occur in primitive aquatic chelicerates (horseshoe crabs)
ii) limb-branch gills in crustaceans are typically exopodites or epipodites, and may be thoracic (most common) or abdominal
b) terrestrial (depend upon diffusion & some inhalation/exhalation):
i) 1 or 2 pair of abdominal 'book lungs' occur in many terrestrial arachnids (diffusion aided by inhalation/exhalation)
ii) spiracle/trachea systems occur in most uniramians, some arachnids and a few crustacea (primarily diffusion)
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Copyright © 2002 by A. Richard Palmer. All rights reserved.
(revised Mar. 16, 2002)