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. {12}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.