Sea spiders are marine arthropods of the order Pantopoda ( ‘all feet’), belonging to the class Pycnogonida, hence they are also called pycnogonids (named after "Pycnogonum", the type genus; with the suffix '). They are cosmopolitan, found in oceans around the world. There are over 1,300 known species, with legs ranging from to over . Most are toward the smaller end of this range in relatively shallow depths; however, they can grow to be quite large in Antarctic and deep waters.