4) Most arthropods have paired, compound eyes (the most spectacular of animal eyes except for vertebrates and cephalopods):
a) composed of multiple ommatidia
i) units with separate cuticular lenses, light-sensitive cells and nerves
ii) 30-35 cells per ommatidium
b) the image is a composite of many spots of light
c) multiple evolutionary origins seems likely: details of ommatidium form vary among groups, they are absent from all chelicerates (except xiphosurans) and from most myriapods
5) 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 and 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)
6) Arthropod coelom is greatly reduced (limited mainly to the gonads)
a) arises embryologically via schizocoely
b) most of the primary coelom fuses with the epineural sinus yielding an extensive hemocoel