What is "arachnid"?

Arachnid refers to a class of joint-legged invertebrates that includes spiders, scorpions, mites, ticks, and harvestmen. They have two main body parts: the cephalothorax and the abdomen, and four pairs of legs attached to the cephalothorax. Arachnids breathe through book lungs, a series of leaf-like membranes arranged like the pages in a book, or through tracheae, tiny tubes that transport oxygen directly to the organs. They have a hard exoskeleton that they must periodically shed to grow. Most arachnids are predators that use venom or powerful pincers to catch their prey. Some species are venomous and can be dangerous to humans, while others are harmless and play important roles in controlling pests. Arachnids can be found in nearly every habitat on Earth, from the deep sea to deserts and forests.