Planarians, a family of worms, or annelids, belonging to the order of turlellaria, which includes also the nemertians. Almost all the order are aquatic, and none are parasitic; they have neither sucking disks nor hooklets, but always vibrating cilia on the integuments. The planarians are soft-bodied, jelly-like, elliptical creatures, found in fresh water and on the seashore; the skin is furnished with numerous cilia, and with cells resembling lasso cells; the intestine, whether simple or branched, never has an anal opening; the water vascular system communicates with the exterior; the nervous system consists of two ganglia, in front of the mouth, united by a cord; there are rudimentary eyes, varying in number from two to sixteen. They are generally called flat worms, to distinguish them from the more elongated nemertians. The experiments of Duges show a remarkable power of repairing injuries, and that by partial division individuals with double heads and tails may be produced. They propagate by eggs deposited and fertilized in the water, by internal buds, or by transverse fission; the young may develop directly into animals resembling the parents, or assume a jointed, bristly annelid structure, as shown by A. Agassiz, which is gradually lost; or the larva may be totally unlike the parent, a wormlike animal separating from a part of the body wall, and the greater portion of the larva body perishing.

The last case is similar to what occurs in some star fishes. Nearly allied forms develop themselves in one or the other of these ways, in the most capricious manner.