Pycnogonids (sea spiders) are peculiar animals of obscure origin. They are thought to have derived directly from proto-chelicerate stock. They are widespread but frequently pass unnoticed due to their normally small sizes (1 –10 mm).
Collosendeis frigida is one of the larger deep-sea species. The prosoma bears four simple eyes and has a head region with a prominent proboscis bearing the mouthparts and a fused leg-bearing thoracic segment. There are also four to six free thoracic segments forming a trunk. The legs are attached to lateral projections of the trunk, which ends in a rudimentary abdomen. Short hooked ventral limbs called ovigers may be used for grooming and in the male for carrying the eggs. Young ‘protonymphons’ look rather like crustaean nauplius larvae. At first they have only three pairs of limbs; they gain the other pairs as successive moults accompany their development into adults.