The Turbellariæ are fresh- and salt-water animals, distinguished by having the whole surface of their bodies covered by cilia, under which in some species there are thread-cells containing six, eight, or a greater number of darts. Most of the members of this tribe have elongated flattened bodies, and move by a sort of crawling or gliding motion over the surface of aquatic plants and animals. Some of the smaller kinds are sufficiently transparent to allow their internal structure to be seen by transmitted light. The mouth, which is situated at a considerable distance from the rounded end of the body, opens into a sort of gullet leading into the stomach, which has no other orifice, but a great number of branching canals are prolonged from it, which carry its contents into every part of the body. A pair of oval nerve-centres are placed near the rounded end of the animal, whence nerves extend to various parts of the body; and near to these there are from two to forty rudimentary eyes according to the species, each of which has its crystalline lens, its pigment layer, nerve bulb, and its cornea. The power of the Planaria to reproduce portions which have been removed is but little inferior to that of the Hydra. _Annelids._ The Annelids are the most highly organized of all the worm tribe. They are exceedingly numerous and varied; some are inhabitants of fresh water, others are terrestrial, but by far the greater number and most highly endowed are marine. They generally have a long, soft, and smooth body, divided or marked by transverse rings into a succession of similar segments. In many the first and last segments are alike; in others the first segment can scarcely be called a head, though it exercises several functions, while in the highest two orders the head is the seat of several senses. On each side of the bodies of the Annelida there are one or two long rows of tufted bristles or feet, which may be regarded as the earliest form of symmetrical locomotive organs. Most of the Annelids have ocelli or eye-specks, and in many of them the head supports soft cylindrical tentacles, which are obviously organs of touch. These worms are divided into four orders, the Suctorial, Terrestrial, Tubercular, and Errantia, or Wandering Worms.[32]