LIFE HISTORY OF THE LIVER-FLUKE. 123 



may also be distinguished here, though the main trunks are 

 less distinctly visible than their ramifications, and can rarely 

 be followed for any great distance. Hence it is impossible to 

 discover whether the system of vessels opens externally. The 

 branches begin with a long narrow infundibulum, in which a 

 flame-shaped cilium is constantly working, as described in the 

 sporocyst. The ciliated infundibula are arranged in two groups 

 on each side of the body ; the anterior group on each side lies 

 a short distance behind the collar, the posterior close to the 

 processes which serve as feet (fig. 15). The ciliated cells do 

 not all lie at the same level beneath the surface, so that occa- 

 sionally two of the infundibula may be seen lying across one 

 another, and sometimes the cells may lie free within the body- 

 cavity, the end of the cell opposite the cilium being connected 

 with the wall by one or more processes (fig. 14). 



The digestive tract is the characteristic structure of the 

 redia, and at once differentiates it from the simple sporocyst. 

 Quite at the anterior end of the body is the mouth surrounded 

 by projecting folds, which may be termed the lips. The trans- 

 verse muscle-fibres are especially well developed in the lips, 

 and assisted by the transverse muscles of the following part of 

 the body-wall, serve as a sphincter muscle in closing the orifice 

 of the mouth. The space within the lips is very small, and 

 leads almost directly into the pharynx, an elliptical muscular 

 organ by means of which the animal draws in and crushes the 

 tissues which serve as food. Its outer surface is formed by a 

 clearly marked limiting membrane, so that it is everywhere 

 distinguished with readiness from the mass of ill-defined cells 

 in which it is embedded, and its cavity is lined by a thickened 

 cuticle. To the pharynx immediately succeeds the digestive 

 sac, a blind tube of very simple structure. Its wall is com- 

 posed of a single layer of clear nucleated cells (fig. 12), sup- 

 ported by a basement membrane, and when it is distended the 

 cells are flattened out till they are little more than discs, in 

 which the nucleus causes a distinct swelling. The digestive sac 

 is seldom more than '3 — '4 mm. long, and may be less than this, 

 but its length differs a good deal, not only in different indivi- 



