Pycnogonids 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 an unusual terminal mouth and several simple eyes on a central tubercle . The head also bears a pair of claws and a pair of ovigers on which the eggs are carried. Pycnogonids feed on soft-bodied invertebrates, in particular cnidarians , sucking at them with their probosces, and larval pycnogonids often live as parasites within cnidarian tissues.
Pycnogonids have almost no fossil record.
Three genera have been found in the Devonian , in the Hunsruck Slate of western Germany. A cast of one of them, Paleopantopus , is shown here. Both pycnogonids and chelicerates have claws on the first appendages and a tubercle with simple eyes, and both lack antennae. However, pycnogonids show so many unusual features, such as the proboscis, reduced abdomen, ovigers, gut diverticulae, and so on, that they may comprise a separate group that probably branched off very early from the arthropod stem. A brief key to the pycnogonids of Woods Hole, Massachusetts is available from the Marine Biological Laboratory at Woods Hole, Massachusetts .