I ANATOMY OF POLYCLADIDA 1 3 



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 * (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. Eespiration 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." 2 After a meal, how- 

 ever, the cell outlines are invisible. Gregarines, encysted Cercariae, 

 and Orthonectida 3 occur parasitically in the gut-branches. 



An excretory system of " flame-cells " and fine vessels has 

 hitherto been seen only by Schultze 4 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. 



Nervous System. The brain, which is enclosed in a tough 

 capsule (Fig. 5, lr\ is placed in front of the pharynx, but some 

 distance behind the anterior margin of the body. It is of an 

 oval shape, subdivided superficially into right and left halves by 

 a shallow depression, and is provided in front with a pair of 

 granular-looking appendages, composed of ganglion -cells from which 

 numerous sensory nerves arise, supplying the eyes and anterior 

 region. Posteriorly the brain gives rise to a chiefly motor, 

 nervous sheath (Fig. 5, nn), which invests the body just within 

 the musculature. This sheath is thickened along two ventral 

 lines (Fig. 5, In) and two lateral lines (n.s), but is very slightly 

 developed on the dorsal surface. Ganglion-cells occur on the 

 course of the nerves, and are particularly large at the point of 

 origin of the great motor nerves. 



Sense Organs. Leptoplana possesses eyes, stiff tactile, mar- 



1 The roof of the peripharyngeal chamber is hence known as the "diaphragm." 



2 See Brandt, Fauna u. Flora d. Oolfes v. Neapel, Monogr. XIII. 1885, p. 65. 



3 See p. 94. 



4 Ver/uindlungen d. mcd. Gesellschaft zu Wiirzburg, iv. 1854, p. 223. 



