The Collaborative International Dictionary of English v.0.48:

Pycnogonida \Pyc`no*gon"i*da\, n. pl. [NL., fr. Gr. ? thick
   crowded + ? knee.] (Zool.)
   A class of marine arthropods in which the body is small and
   thin, and the eight legs usually very long; -- called also
   Pantopoda.
   [1913 Webster]

   Note: The abdomen is rudimentary, and the triangular mouth is
         at the end of a tubular proboscis. Many of them live at
         great depths in the sea, and the largest of them
         measure two feet across the extended legs.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Pycnogonida
    n 1: sea spiders [syn: Pycnogonida, order Pycnogonida]