Members of class Pycnogonida are commonly known as sea spiders. Most sea spiders have four pairs of legs. They feed with a sucking proboscis on algae and other invertebrates and are found in oceans all over the world. Arachnids are predominantly terrestrial, and most are carnivorous, with the digestion of prey starting outside the body.
Arachnids are an ancient group, their fossil records dating back to the Carboniferous period.