POLYCLADIDA 



with eggs forms a dark margin round the latter (Figs. 1 and 

 2, nt). The ventral surface is whitish, and through it the 

 " pharynx," a frilled protrusible structure, may be dimly observed. 

 The "mouth," 1 through which the pharynx at the time of feeding 



<2o 



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. x 5. 



is thrust out (Fig. 2, mo), is almost in the centre of the ventral 

 surface. Behind this, a white, Y-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, $). 



1 Since no food, but only the pharynx, passes through this " mouth," the term 

 is unfortunate. Moreover the true mouth is the aperture placing the stomach in 

 communication with the pharynx (Fig. 5, gin). 



