12 PLATYHELMINTHES TURBELLARIA . chap. 



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 (ra.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 Zeptoplana exerts if sud- 

 denly 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 com- 

 posed 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 Zeptoplana possesses in such a high degree. Pig- 

 ment 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 



