Sea spiders resemble true spiders, but sea spiders have an unusual probiscis which they use for sucking up their food. They are all aquatic animals and have about 1,000 species. They live in intertidal regions as well as the deep sea, and although most species are fairly small, some can reach up to 70 cm in diameter. They eat hydroids and other cnidarians, and also worms. Others eat algea and sea anemones as well. Most usually crawl around the bottom of the sea, though some are able to swim. They usually have four pairs of legs, though the males have an extra pair of legs called ovigers to carry the eggs.