Pycnogonids are seldom seen because they are pale or cryptically colored. Most pycnogonids are about 0.39 in (1 cm) or less in size, but some deep-sea forms reach up to 27.5 in (70 cm) across between leg tips. They have extremely reduced bodies in which the abdomen has almost disappeared, while the legs are long and clawed. The head has a long proboscis with a terminal mouth and a single four-part eye on a central stalked tubercle.
The surface area of the thin body and legs allows diffusion of gases and wastes, since pycnogonids lack specialized respiratory or excretory structures. Digestion occurs in the gut, which sends branches into the long legs. Most species have four pairs of walking legs, but some have five or six pairs, with the reproductive organs located in the joints of the legs. The males, and often the females, have ovigerous legs in addition to the walking legs. These legs are used for holding and carrying eggs during the breeding season and, in females, for cleaning and grooming outside of the breeding season. The body is contained in and supported by a non-calcareous exo-skeleton.
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