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.
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.
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.)
At low water Leptoplana may be found buried in mud or on the under surface of stones, in pools where darkness and dampness may be ensured till the return of the tide. It is, however, by no means easy to detect and remove it from the encrusting Polyzoa, Ascidians, or Sponges with which it is usually associated. The flat, soft, unsegmented body is so closely appressed to the substratum that its presence is usually only betrayed by its movement, an even gliding motion of the mobile body, which suggested the apt name "la pellicule animée" to Dicquemare. The creeping surface is called ventral, the upper one dorsal, and as the broader end of the body always goes first, it is anterior as opposed to the more pointed posterior extremity. With a lens the characters shown in Figs. 1 and 2 may be observed. The eyes are seen as black dots near the anterior end, and are placed at the sides of a clear oval space, the brain. Along the transparent margin of the body, the ends of the intestinal branches may be seen. These ramify from a lobed stomach or main-gut, and should the specimen be mature, the "uterus" loaded with eggs forms a dark margin round the latter (Figs. 1 and 2, ut). The ventral surface is whitish, and through it the "pharynx," a frilled protrusible structure, may be dimly observed. The "mouth,"[[16]] through which the pharynx at the time of feeding is thrust out (Fig. 2, mo), is almost in the centre of the ventral surface. Behind this, a white, V-shaped mark (vd) indicates the ducts of the male reproductive organs, and still further back is the irregular opaque mark of the "shell-gland," by which the egg-shells are formed (Fig. 2, ♀).
Fig. 3.—Leptoplana tremellaris in the act of swimming. A, Seen from the right side during the downward stroke (the resemblance to a skate is striking); B, from above, showing the upward stroke and longitudinal undulations of the swimming lobes; C, side view during the upward stroke; D, transverse sections of the body during the strokes. × 5.