Sea spiders are a separate class of arthropods, the Pycnogonida.  They are
generally classified in the subphylum Chelicerata, which also includes
arachnids, harvestmen, scorpions, pseudoscorpions, horseshoe crabs, and a
number of less well known small groups.  Therefore they are not closely
related to isopods, which are crustaceans, and therefore in a different
subphylum.  Actually they are more closely related to true spiders than to
crustaceans, though just how closely related they are is not well known.
Many of them feed on small colonial invertebrates like hydroids and
bryozoans.  Bizarre is certainly a descriptive term for an animal which has
almost no body, and is practically all legs.
Paul M.