PLATYHELMIA. 



29 



spines prevent the animal from slipping back as it makes its way 

 along the bile-ducts of its host. 



The digestive and other organs are imbedded in a mass of 

 tissue, which, since it supports the various parts and connects 

 them together, may be termed connective tissue. Polyhedral 

 nucleated cells are the main constituent of this tissue in the 

 Liver-fluke, but there is also a certain amount of non-cellular 

 fibrous material between these. 



2. Digestive Organs (Fig. 5). The mouth leads into a short 

 tube which quickly forks, the two limbs of the fork passing back 

 and giving off numerous branching processes, all of which end 

 blindly. There is no anus. 



The oval mouth (0) is situated in the middle 

 of the anterior sucker, and leads into a small 

 mouth-cavity, which is followed by the oval, 

 thick-walled pharynx that passes into a very 

 short, straight, delicate tube, the gullet or 

 oesophagus, this again opening into a bifurcated 

 intestine. Each half of this is a thin-walled, 

 fairly wide tube (D), running back to the end 

 of the body, and situated near the middle line. 

 A number of small pouches project from its 

 inner side, whilst a large number of pouches, 

 mostly much branched, extend from its outer 

 side to the edge of the body. 



Special muscles i.e., bands of contractile 

 fibres are connected with the pharynx. A 

 sheath of such fibres, the protractor muscle, 

 closely surrounds this organ, and, on the other F - 5 _Ip ISTOMA 

 hand, is connected with the anterior sucker, ( from Claus after 

 while retractor muscles slant back from the 

 pharynx to the dorsal wall of the body. 



Histology. The pharynx is lined by a con- 

 tinuation of the general cuticle. Its wall is very 

 muscular, the fibres taking various directions. 

 The gullet and intestine have very thin walls, 

 consisting of an epithelial layer of cells, external 

 to which is a structureless membrane. The term epithelium is 

 applied to membranes, formed by one or more layers of cells,, 

 which cover external and line internal surfaces. The epithelium 



Leuckart) Ali- 

 mentary canal. 

 O, Mouth, a 

 short distance 

 behind which is 

 ventral sucker. 

 D, right limb of 

 intestine. 



