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