5e:Spider
by John C. Stennis Space Center, National Aeronautics and Space Administration (NASA)
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Spiders (order 
Araneae) are air-breathing arthropod s that have eight legs and chelicerae with fangs able to inject venom . They are the largest order of arachnid s… Spiders are found worldwide on every continent except for Antarctica , and have become established in nearly every habitat …
Anatomically, spiders differ from other arthropods in that the usual body segments are fused into two tagmata , the cephalothorax and abdomen , and joined by a small, cylindrical pedicel . Unlike insect s, spiders do not have antennae . In all except the most primitive group, the Mesothelae , spiders have the most centralized nervous systems of all arthropods, as all their ganglia are fused into one mass in the cephalothorax. Unlike most arthropods, spiders have no extensor muscles in their limbs and instead extend them by hydraulic pressure.
Their abdomens bear appendages that have been modified into spinneret s that extrude silk from up to six types of glands. Spider web s vary widely in size, shape and the amount of sticky thread used. It now appears that the spiral orb web may be one of the earliest forms, and spiders that produce tangled cobwebs are more abundant and diverse than orb-web spider s.
List of Spiders
7 Spiders
