The discovery of particularly large sea spiders, with a length up to 25 cm (10 inches), in the polar seas both Arctic and Antarctic is causing puzzlement in the scientific world because of the difficulty in finding an explanation to these animals’ size. They’re actually called pycnogonids, but are also known as Pantopoda, and are arthropods belonging to the class Pycnogonida, which means that they are not really spiders.