by von Graff. The Turbellaria are divided into: (1) _Polycladida_, marine forms with multiple intestinal branches; (2) _Tricladida_, marine, fresh-water, and terrestrial Planarians with three main intestinal branches; (3) the _Rhabdocoelida_, as varied in habit as the Triclads, but possessing a straight and simple or slightly lobed, intestine. A detailed description of an example of the Polyclads, and then a comparative account of each division, will now be given. [Illustration: FIG. 1.—_Leptoplana tremellaris_ O. F. M. Seen from the dorsal surface. The alimentary canal runs down the middle line and sends branches to the margin of the body. × 6.] TURBELLARIA. I. POLYCLADIDA. _Description of Leptoplana tremellaris._ APPEARANCE AND HABITS.—An account of the Polyclad Turbellaria may be fitly prefaced by a description of a very common representative, _Leptoplana tremellaris_, so called on account of the thin, flat body which executes when disturbed, quivering or tremulous swimming movements. {8}Like all Polyclads, _Leptoplana_ is marine. It is probably found on all European shores, northwards to Greenland and southwards to the Red Sea, while vertically it ranges from the littoral zone down to fifty fathoms. There is, however, an apparently well-marked difference between the littoral specimens, which vary from three-quarters to one inch in length, are brownish in colour and firm in consistency, and the more delicate examples half an inch long, white with a brown tinge, which occur in deeper water. [Illustration: FIG. 2.—_Leptoplana tremellaris._ Three-quarters view from the ventral surface. The pharynx (_ph_) is widely protruded through the month (_mo_) as in the act of attacking prey. _br_, Brain with nerves, close to which are the four groups of eyes; _mg_, stomach; _mgc_, "marginal groove"; _pe_, penis; _sc_, sucker; _ut_, uterus; _vd_, vasa deferentia; ♀, female genital aperture surrounded by the shell-gland; ♂, male aperture. (Semi-diagrammatic, and × 6.)]