They are now extinct. Their abdomen is thin, long, and pointed, hence the class name. They are marine, typically found in intertidal areas. They have simple eyes on top of their cephalothorax. The first pair of appendages is modified as chelicerae, small pincer-like mouthparts, which are further modified as fangs in spiders.
Arachnids have four pairs of walking legs.
Occasionally there are other appendages, like the spinnerets in spiders. Arachnids breathe via book lungs . Crustaceans have the head and thorax combined into one body region, the cephalothorax, as well as an abdomen. Centipedes and millipedes don't really have 100 or 1000 legs, but they do have lots. Note that their mandibles are modified appendages and are NOT homologous to our mouth or teeth.