The 75000 species within this subphylum belong to three diverse classes. The Classes Merostomata and Pycnogonida include relatively minor, aquatic organisms, the Class Arachnida comprises the vast majority of species (74000 + ) and includes spiders, scorpions, mites and ticks.
The chelicerate body comprises an anterior prosoma or cephalothorax, with 6 pairs of appendages and a posterior abdomen, containing the respiratory and sexual organs. The first pair of appendages are pre-oral, pincer-like chelicerae, the second pair are leg-like pedipalps. There are generally four pairs of legs, the anterior ones sometimes function as feelers or copulatory organs. The abdominal appendages have been reduced or modified as in spiders into spinnerets. The appendages are unbranched.