The rays, which form the order Batoidea, are divided into seven families. The largest are the mantas, also called devil rays and devilfish (family Mobulidae). These are top-swimming forms which may weigh up to 3,000 lb (1360 kg), with a width of up to 22 ft (7 m). Unlike most rays, mantas are filter-feeders; the manta uses a pair of horns at the front of the head to drive small prey into its mouth; there the prey is caught in a strainer and swallowed, the water passing out through the manta's gills. Electric rays, or torpedos (family Torpedinidae), have electric organs in their wings that generate electric current, used to immobilize prey and for defense. The current is strong enough to stun humans, and it is said that the ancient Greeks used these fish for shock therapy. Skates (family Rajidae), which are sometimes caught for food, are bottom dwellers; some species have electric organs in their tails. The stingrays, or whiprays (family Dasyatidae), have rows of spines along their tails, which are generally much longer than their bodies. The stingray inflicts wounds by lashing with its tail; the spines contain a poison that causes pain and can be fatal to humans. Most of the eagle rays and bat rays (family Mylobatidae) bear a single poison spine on the tail. The guitarfishes (family Rhinobatidae) are sharklike in form, having well-developed tails used for swimming and smaller pectoral fins than most rays; however, the fins are attached, as in all rays, above the gills, giving these fishes a broad-headed appearance. Sawfishes (family Pristidae) are similar in body form, but have long, flat snouts with a row of toothlike projections on either side. Some species reach a total length of 20 ft (6 m), with snouts 6 ft (1.8 m) long and 1 ft (30 cm) wide. They use these ponderous weapons to slash and impale small fishes and to probe in the mud for burrowing animals. Sawfishes, which are endangered globally, should not be confused with saw sharks, which are true sharks.