Sea spiders appear to be a sort of marine "spider," but in fact their relationships are enigmatic. They may represent a very early branching of the chelicerate lineage. There are approximately 1000 described species of pycnogonids, all of which are marine.
Pycnogonids can be found from the intertidal regions to depths of around 7000 m. Most are small, but a few deep-sea forms reach up to 70 cm diameter across the legs. They feed by sucking juices from soft-bodied invertebrates through a long proboscis.
Pycnogonids vaguely resemble spiders, with small bodies and relatively long, hinged legs. Unique characteristics include an unusual proboscis, which varies in size and shape among species, but amounts to a chamber with an opening at the distal end (the true mouth lies between the proboscis chamber and the esophagus). The body itself is not divisible into neatly- organized tagmata or regions as it is in most other arthropods. An anterior region bears, besides the proboscis, three or four pairs of appendages, including the first pair of walking legs. Special appendages called ovigers when present make up the four pair; these play a role in the brooding of young and are used in cleaning. Following the first segment is a series of segments making up a "trunk," each segment bearing a pair of walking legs. A terminal segment includes a tubercle that projects dorsally and an anus. Some species have more than four pairs of walking legs. Pycnogonids are also unique in bearing multiple gonopores, found on the second segment of some or all of the walking legs.