Source : Webster's Revised Unabridged Dictionary (1913)

Sea spider \Sea" spi"der\ (Zo["o]l.)
   (a) Any maioid crab; a spider crab. See {Maioid}, and {Spider
       crab}, under {Spider}.
   (b) Any pycnogonid.

Source : WordNet®

sea spider
     n : any of various small spider-like marine arthropods having
         small thin bodies and long slender legs [syn: {pycnogonid}]