Definition of Pycnogonida
: a class of marine arthropods that superficially resemble spiders with the body relatively very thin and small, the legs usually excessively long and slender, and the abdomen rudimentary, that have typically seven pairs of appendages of which the posterior four pairs are legs and contain diverticula of the intestine and reproductive organs and of which the first two or three pairs are often absent, a triangular mouth at the end of a tubular proboscis, no organs of respiration, and separate sexes, and that produce young which usually pass through a metamorphosis
