Claw-horns split from other arthropods by mid-Cambrian seas
Chelicerates—sea spiders, horseshoe crabs, and arachnids—form one of arthropods' two great living branches beside Mandibulata. Their name fuses Greek words for claw and horn. Fossils such as Habeliida and Mollisonia show the split by mid-Cambrian; living diversity soars past 77,000 described species, mostly spiders and kin.
Only a handful of chelicerates stayed in the sea: four species of horseshoe crab and perhaps 1,300 sea spiders, if those truly belong in the group. On land the lineage exploded, with more than 77,000 air-breathing species described and possibly half a million still unnamed. Megachelicerax joins the mid-Cambrian stem forms, and extinct offshoots include the eurypterids, or sea scorpions.
The body divides into two sections, a front prosoma and a rear opisthosoma of thirteen or fewer segments, though mites have lost the obvious split. Two pairs of appendages sit nearest the mouth. The chelicerae, the claw horns behind the name, usually have three sections, with the claw formed by the third and a stiff extension of the second; spiders have only two, the second shaped into a fang. Behind them come the pedipalps, simple leg-like limbs in some groups but specialised for sensing or seizing prey in others. Like every arthropod they wear a chitin cuticle that cannot stretch, so they must moult to grow.
Although the lineage began as hunters, it now includes parasites, herbivores, scavengers and decomposers. Harvestmen can digest solid food, but most others take their meals as liquid. Horseshoe crabs fertilise eggs externally, while land species use internal, usually indirect fertilisation, often after elaborate courtship. Venom evolved separately in spiders, scorpions and pseudoscorpions, or four times if tick secretions count, and researchers are testing some spider and scorpion venoms against conditions from cancer to erectile dysfunction. Blood flows through a large body cavity called a hemocoel, and land-dwellers typically breathe with both book lungs and tracheae.
Source: Chelicerata
