Leptoplana employs two kinds of movement, creeping and swimming. Creeping is a uniform gliding movement, caused by the cilia of the ventral surface, aided perhaps by the longitudinal muscular layers of this surface, and is effected on the under side of the "surface-film" of water almost as well as on a solid substratum. Swimming is a more rapid and elegant movement, employed when alarmed or in pursuit of prey. The expanded fore-parts of the body act as lobes, which are flapped rapidly up over the body and then down beneath it, undulations running rapidly down them from before backwards. The action in fact is somewhat similar to that by which a skate swims, a resemblance pointed out long ago by Dugès[[17]] (Fig. 3).
We have few direct observations on the nature of the food of Leptoplana, or the exact mode by which it is obtained. Dalyell,[[18]] who observed this species very carefully, noticed that it was nocturnal and fed upon a Nereis, becoming greatly distended and of a green colour after the meal, but pale after a long fast. Keferstein[[19]] noticed a specimen in the act of devouring a Lumbriconereis longer than itself, and also found the radulae of Chiton and Taenioglossate Molluscs in the intestine. That such an apparently weak and defenceless animal does overpower large and healthy Annelids and Mollusca, has not hitherto been definitely proved. Weak or diseased examples may be chiefly selected. The flexible Leptoplana adheres firmly to its prey, and the rapid action of the salivary glands of its mobile pharynx quickly softens and disintegrates the internal parts of the victim. The food passes into the stomach (Fig. 2, mg), and is there digested. It is then transferred to the lateral branches of the intestine, and, after all the nutritious matters have been absorbed, the faeces are ejected with a sudden contraction of the whole body through the pharynx into the water.
Leptoplana probably does not live more than a year. In the spring or summer, batches of eggs are laid and fixed to algae or stones by one individual, after having been fertilised by another. Young Leptoplana hatch out in two to three weeks, and lead a pelagic existence till they are three or four millimetres in length. In late summer, numbers of such immature examples may be found among sea-weeds and Corallina in tide pools. In the succeeding spring they develop first the male and then the female reproductive organs.
Fig. 4.—Portion of a transverse section of Leptoplana tremellaris in the hinder part of the body. × 100. bm, Basement (skeletal) membrane; cil, cilia; d.m, diagonal muscles; d.v.m, dorso-ventral muscles; ep, epidermis; f.p, food particles; l.g, lateral intestinal branches cut across; l.m ext, external, and l.m int, internal longitudinal muscle layers; m.c, glandular (mucous) cells; md, their ducts; N, longitudinal nerve; Nu, nuclei of the intestinal epithelium; ov, ovary; ovd, oviduct; par, cells of the parenchyma; r.d, vasa deferentia, with spermatozoa; rm, circular musculature; rh, rhabdites; sh, cells of the shell-gland; te, testes; ve, vasa efferentia; y.c, "yellow cells." (After Lang.)
Anatomy of Leptoplana tremellaris.—Leptoplana may be divided into corresponding halves only by a median vertical longitudinal plane. The body and all the systems of organs are strictly bilaterally symmetrical. Excepting the cavities of the organs themselves, the body is solid. A connective "parenchyma" (Fig. 4, par) knits the various internal organs together, while it allows free play of one part on another. These organs are enclosed in a muscular body-wall, clothed externally by the ciliated epidermis, which is separated from the underlying musculature by a strong membrane (Fig. 4, bm), the only skeletal element in the body.
Body-Wall.—The epidermis (Fig. 4, ep) is composed of a single layer of ciliated cells, containing small, highly refractive, pointed rods or "rhabdites" (rh), and gives rise to deeply-placed mucous cells (m.c), which are glandular and pour out on the surface of the body a fluid in which the cilia vibrate. The tenacious hold on a stone which Leptoplana exerts if suddenly disturbed, or when grasping its prey, is probably due to the increased glutinous secretion of these glands, aided perhaps by rhabdites, which on such occasions are shot out in great numbers. The basement membrane is an elastic skeletal membrane composed of stellate cells embedded in a firm matrix. It serves chiefly for the origin and insertion of the dorso-ventral muscles (d.v.m). Under the basement membrane lies a very thin layer of transverse muscular fibres (Fig. 4, rm), which are, however, apparently absent on the ventral surface. Then follows a stout layer of longitudinal fibres (l.m ext), and beneath this a diagonal layer (d.m), the fibres of which intersect along the median line in such a way that the inner fibres of one side become the outer diagonal fibres of the other. Lastly, within this again, on the ventral surface, is a second stout longitudinal layer (l.m int). The sucker (sc, Figs. 2 and 5) is a modification of the body-wall at that point. In addition to the dorso-ventral muscles, there exists a complex visceral musculature regulating the movements of the pharynx, intestine, and copulatory organs.
Parenchyma.—The spaces between the main organs of the body are filled by a tissue containing various kinds of cells, salivary glands, shell-glands, and prostate glands. Besides these, however, we find a vacuolated, nucleated, thick-walled network, and to this the word parenchyma is properly applied. Besides its connective function, the parenchyma confers that elasticity on the body which Leptoplana possesses in such a high degree. Pigment cells are found in the parenchyma in many Polyclads.
Digestive System.—The general arrangement of this system may be seen in Figs. 2, 5, and 7; and may be compared, especially when the pharynx is protruded, as in Fig. 2, with the gastral system of a Medusa. The "mouth" (there is no anus) is placed almost in the centre of the ventral surface. It leads (Fig. 7, B, phs) into a chamber (the peripharyngeal space) divided into an upper and a lower division by the insertion of a muscular collar-fold (the pharynx, ph), which may be protruded, its free lips advancing, through the mouth (Fig. 2), and is then capable of enclosing by its mobile frilled margin, prey as large as Leptoplana itself. The upper division of the chamber communicates by a hole in its roof[[20]] (the true mouth, Figs. 5 and 7, g.m) with the cavity of the main-gut or stomach (m.g), which runs almost the length of the body in the middle line, forwards over the brain (Fig. 5, up). Seven pairs of lateral gut-branches convey the digested food to the various organs, not directly however, but only after the food mixed with sea-water has been repeatedly driven by peristalsis first towards the blind end of the gut-branches and then back towards the stomach. Respiration is probably largely effected by this means. The epithelium of the intestine (Fig. 4, l.g) of a starving specimen is composed of separate flagellated cells frequently containing "yellow cells."[[21]] After a meal, however, the cell outlines are invisible. Gregarines, encysted Cercariae, and Orthonectida[[22]] occur parasitically in the gut-branches.
An excretory system of "flame-cells" and fine vessels has hitherto been seen only by Schultze[[23]] in this species, which will not, however, resist intact the compression necessary to enable the details to be determined. They are probably similar to those of Thysanozoon described on p. [25].