Leaf-like or vermiform Scolecids, rarely parasitic, with a mouth and alimentary canal, and sometimes a body-cavity; integument ciliated. Sexes united or distinct.
The members of this order are almost all aquatic, and are all non-parasitic; thus differing entirely from the animals which compose the two preceding orders. Their external surface is always and permanently ciliated, and they never possess either suctorial discs or a circlet of cephalic hooklets. A "water-vascular system" is present, opening externally by one or more apertures, or appearing to be entirely closed in the adult (Nemertida). The alimentary canal is embedded in the parenchyma of the body (Planarida), or is freely suspended in a "perivisceral cavity" (Nemertida). The intestine is either straight or branched, and a distinct anal aperture may, or may not, be present. The nervous system consists of ganglia situated in the fore-part of the body, united to one another by transverse cords, and sending filaments backwards.
The Turbellaria are divided into two sections, termed respectively the Planarida and the Nemertida.
The Planarians (fig. 117) are mostly ovoid or elliptical in shape, flattened and soft-bodied.
Fig. 117. - Morphology of Turbellaria. 1. Planaria torva (Muller) : m Mouth; g Nerve-ganglion; e Eyes; ov Ovary; t Testis; gn Genital opening. 2. Planaria lactea, showing the branched (dendrocoel) intestine. 3. Microscopic larva of A lau-rina, a marine Turbellarian. 4. Pilidium, the "pseudembryo" of a Nemertid : a The alimentary canal; b Rudiment of the Nemertid.
They are for the most part aquatic in their habits, occurring in fresh water, or on the sea-shore, but occasionally found in moist earth. The integument is abundantly provided with vibratile cilia, which subserve locomotion, and it also contains numerous cells, which have been compared to the "cnidae," or nettle-cells, of the Coelenterata. There is always a considerable portion of the body situated in front of the mouth, constituting the so-called "prae-oral region," or "prostomium;" and this is often modified into a singular protrusible and retractile organ, called the "proboscis," the exact use of which is not known. The mouth opens into a muscular pharynx, which is often evertible; and the intestine may be either straight or branched, but always terminates caecally behind, and is never provided with an anal aperture. The "water-vascular system" communicates with the exterior by two or more contractile apertures. The nervous system consists of two ganglia, situated in front of the mouth, united by a commissure, and giving off filaments in various directions. Pigment-spots, or rudimentary eyes, from two to sixteen in number, are often present, and are always placed in the prae-oral region of the body. The male and female organs are united in the same individual, and the process of reproduction may be either sexual, by means of true ova, or non-sexual, by internal gemmation or transverse fission.
The Planarians have been divided into two sections, as follows: