THE LIVER FLUKE 



121 



^-e.v 



RESPIRATION 



There is no evidence that the Hver fluke is anaerobic (p. ii). 

 Experimentally a reduced oxygen tension shortens the life of the 

 worms, and measurements show 

 that bile is nearly saturated with 

 oxygen. 



EXCRETORY SYSTEM 



The excretory system (Fig. 85) 

 lies in the parenchyma. It consists 

 of a meshwork of tubules joining 

 into a main duct which lies in the 

 middle line, from a point about a 

 quarter of the length of the body 

 behind its front end to the ex- 

 cretory pore at the hind end. 

 The ultimate branches of the 

 tubules are very fine and end, at 

 least in the larva, in little struc- 

 tures known as flame cells or 

 solenocytes (Fig. 86). These are 

 minute vesicles containing a few 

 long cilia which keep up a flicker- 

 ing movement like that of a flame 

 and so perhaps drive towards the 

 main duct the fluid secreted into 

 the vesicle by its walls. Each 

 vesicle has a nucleus and may 

 be regarded as a hollow cell. It is 

 connected with its fellows by fine 

 protoplasmic processes which are 

 said to be hollow. The flame 

 cells are said to disappear in the 



Fig. 85. — The structure of a liver 

 fluke. — After Sommer. From the 

 ventral surface. The branched 

 gut {g.) and the lateral nerve 

 [l.n.) are shown to the left of the 

 figure, the branches of the 

 excretory vessel {e.v.) to the 

 right. 



Position of cirrus sac ; eg., lateral head 

 ganglion ; m., mouth ; ph., pharyn.\ ; v.s., 

 ventral sucker. An arrow indicates the 

 excretory aperture. 



c.s. 



cercaria stage (p. 126) and to be 



invisible in the adult. The whole system is derived from the 



ectoderm and is called a protonephridium. 



The fluid in the tubules contains much fat, and is almost 

 continuously squeezed out through the excretory pore by con- 

 tractions of the body. Fat is an unusual excretory product, but 



M.Z.— 5 



