Short answers, pulled from the story.
The word arachnid comes from the Greek arachne, meaning spider, a name bestowed upon the class after the tragic myth of Arachne, a human weaver who challenged the goddess Athena to a contest of skill and was transformed into a spider for her hubris.
There are over 110,000 described species of arachnids, with spiders alone accounting for 51,000 of them.
Most arachnids lack extensor muscles in the distal joints of their appendages and rely on hydraulic pressure to extend their limbs by pumping hemolymph into the joints to create the necessary force for movement.
Arachnids utilize either tracheae or book lungs to exchange gases with the air, with book lungs functioning as an internal series of vascular lamellae adapted for air breathing.
A fossil arachnid in 100 million year old amber from Myanmar, Chimerarachne yingi, lived during the Cretaceous period and possesses spinnerets to produce silk and also has a tail.
The Araneae, or spiders, are the most numerous with 51,000 species, while the Opiliones, or phalangids, harvestmen, or daddy-long-legs, have 6,700 species.