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The appearance of the long-legged sea spiders are very similar to the "true", terrestrial spiders. There are usually four, but some species have six pair of legs. The body is slim. A respiratory system is not necessary, due to the relatively large body surface and small volume. The required oxygen diffuses through the surface. At the back of the head there is one or two pair of eyes. One pair is usually directed forwards, the other backwards. Some deep water species may reach a length of 90 cm, but many species are in the range of 3 to 30 mm (30).