Chelicerata
	Merostomata (horseshoe crabs)
	Pycnogona (sea spiders)
	Arachnida spiders
	Arachnida scorpions
	Arachnida solifugae (sun spiders, wind scorpions)
	Arachnida palpigradi (microwhip scorpions)
	Arachnida Uropygi (whip scorpions)
	Arachnida Amblypygi (whip spiders)
	Arachnida Schizomida, short-tailed whip scorpions
	Arachnida pseudoscorpiones
	Arachnida mites, watermites
	Arachnida ticks
	Arachnida opiliones, harvestmen

Class Pycnogona

These are the sea spiders, over 1000 species have been described; all are marine. They are usually less than 6 cm in body length, but can have legs over 30 cm long. They have four pairs of walking legs, and the males have an additional pair of small legs called ovigers (see Nymphon rubrum right) which they use for carrying the developing eggs. There can be 1000 eggs in an egg mass. Most have two pairs of simple eyes and no gaseous exchange system, so diffusion of gases and wastes occurs over the body surface. Their mouth is located at the end of a sucking proboscis.

They crawl slowly over Cnidaria and Bryozoa, although some species can swim by flapping their legs. They are carnivorous and feed on the juices of Cnidarians, Bryozoa, Porifera and other soft-bodies animals. They are found in all oceans, but are most abundant in Polar regions.