(also Pycnogonida; sea spiders), a class of invertebrates of the phylum Arthropoda. Body length, 0.8 mm to 5 cm. The body consists of a large proboscis, a segmented trunk, and a short caudal section. The first pair of limbs are chelate and serve to capture food. The second and third pairs of limbs resemble tentacles and are for touch (in males, the third pair is for carrying the young). The next four to six pairs are walking legs, with spans from 1.4 mm to 50 cm; they contain sex glands and a branch of the stomach. The nervous system is in the form of a chain. On the back is an eye tubercle with four eyes.

There are about 500 species of sea spiders. They are marine benthic predators: adult sea spiders and their larvae feed on the soft tissues of Actinia, Hydrozoa, Ascidia, Echinodermata, and Mollusca. Sea spiders are found from the littoral zone to depths of 7,500 m.