Sea Spiders (Pycnogonida)

The Pycnogonida (Gr. pyknos, thick, dense + Gr. gony, knee) are a class of small, primarily benthic marine arthropods often called sea spiders because they superficially resemble the true, terrestrial spiders. Like true spiders, pycnogonids possess an anterior pair of chelicerae followed by a pair of pedipalps and four pairs of long, slender legs. However, unlike spiders or any other arthropod, pycnogonids possess an unsegmented anterior proboscis, special brooding appendages (ovigers) in the male, male brooding behavior, an unusual body form with poorly-defined regions, a unique body-coxa joint, multiple gonopores, and polymerous species with 10 or 12 walking legs instead of the usual eight. The cephalothorax is more slender and the abdomen is much smaller than that of true spiders. Unlike true spiders, sea spiders do not possess a specialized respiratory system. Much of the body cavity is filled with the digestive and reproductive systems, both of which extend far into the legs – an...