sea spider, common name for members of the class Pycnogonida, long-legged, rather spiderlike organisms of the subphylum Chelicerata, widely distributed in marine waters. Most are tiny, from 1 to 9 mm (0.04–0.36 in.), and live in littoral regions, crawling about over the surface of sessile animal colonies or seaweeds. Some live on or in clams. There are deep-sea forms, some becoming quite large; Colossendeis colossea has a leg span of nearly 2 ft (91 cm). Their unusual body form makes their relationships to other arthropods obscure. Nearly all of the body is composed of the anterior region (prosoma); a tiny tubular posterior region (opisthosoma) projects behind. A large proboscis is used to suck in food. At the base of the proboscis is a pair of modified appendages (chelicera) used to pick off bits of food and hold them in front of the mouth. The next appendages are a pair of leglike pedipalps, followed by a pair of specialized legs used by the male to carry eggs until they hatch. Four to six pairs of walking legs follow. The reduced body size has led to the extension of the organs into the appendages. They lack gills or lungs; oxygen is absorbed from seawater through diffusion, using pores in the exoskeleton in the largest sea spideers. Members of this class are relatively common and widely distributed; some 1,300 species are known. Sea spiders are classified in the phylum Arthropoda, subphylum Chelicerata, class Pycnogonida.
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