Pycnogonidae, a very remarkable group of Arthropod animals, perhaps intermediate between Crustaceans and Arachnids. The body consists of a fused cephalothoracic region, three free thoracic segments, and a rudimentary abdomen. The head usually bears a tubular proboscis, a pair of mandibles, a pair of slender palps, and a pair of egg-carrying legs; but mandibles and palps may be absent, and the egg-carrying legs are sometimes restricted to the males. Besides these there are four pairs of clawed limbs, into which prolongations of the gut extend. There is a dorsal heart; respiration is effected through the skin. The males usually carry the eggs. There is a metamorphosis in development. The pycnogonidae are sometimes called 'sea-spiders' and also Pantopoda. They are all marine, and some of them live among algae, or are to be found under stones on the beach, whilst others are dredged from deep water. They seem to feed by sucking other animals. See Hoek, Challenger Report (iii. 1880); and Dohrn, Fauna d. Golfes v. Neapel (iv. 1881).
