Sea spiders are marine arthropods of the order Pantopoda (lit. ‘all feet’), belonging to the class Pycnogonida, hence they are also called pycnogonids (named after Pycnogonum, the type genus; with the suffix -id). They are cosmopolitan, found in oceans around the world. There are over 1,300 known species, with legs ranging from 1 mm (0.04 in) to over 70 cm (2.3 ft). 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. Although "sea spiders" are not true spiders, or even arachnids, their traditional classification as chelicerates would place them closer to true spiders than to other well-known arthropod groups, such as insects or crustaceans. This is in dispute, however, as genetic evidence suggests they may be the sister group to all other living arthropods.