An order of the Turbellaria (of the phylum Platyhelminthes) known commonly as planaria, which are several millimeters to 20 in. (50 cm) or more in length. They have a diverticulated intestine with a single anterior branch and two posterior branches separated by a plicate pharynx or pharynges. Rhabdites are numerous and, except in cave planarians, two to many eyes are present. The much branched protonephridial tubules form a network with numerous nephridiopores on each side of the body. The female reproductive system includes a single pair of small anteriorly located ovaries, numerous minute yolk glands arranged in clusters along either side of the body, the common ducts, the female antrum, and usually one or more bursae. The male system has several to many testes, which are lateral in position and connected with a single sperm duct on each side. These ducts empty either directly or after fusion into the copulatory organ which lies in the male antrum. Following copulation and mutual insemination, capsules containing several fertilized eggs are attached to objects in the water and hatch in 2 or more weeks into young worms. Asexual reproduction by fission is common in forms such as the cosmopolitan Dugesia tigrina which has been much used in studies on regeneration. Fragmentation and regeneration is the usual method of reproduction in some land planarians such as Bipalium kewense, an exotic species which has become established through much of the southern United States. The marine planarian Bdelloura candida is a commensal on the horseshore crab.