POLYCLADIDA 



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

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

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

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



FIG. Z.Leptoplana tremeUaris in the act of swimming. A, Seen from the right side 

 during the downward stroke (the resemblance to a skate is striking) ; B, from 

 alwve, showing the upward stroke and longitudinal undulations of the swimming 

 lol>es ; 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, V-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, 9). 



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, gi). 



