CHAP, v FASCIOLA 161 



branches on its outer side and these, branching again, fill up a great 

 part of each half of the body with blindly ending branches. 



There is no trace of coelomic body-cavity, the interval between the 

 alimentary canal and the body-wall being filled up with a cellular sponge- 

 work (parenchyma), with fluid in its meshes. Connected with this is the 

 excretory system very different from the corresponding system of the 

 annelids. Running along the median axis of the body in its posterior 

 two-thirds is a wide tube,- the main excretory duct (Fig. 74, H, ). This 

 opens to the exterior by a pore at the hinder edge of the fluke. Com- 

 municating with the main duct is a complicated network of fine excretory 

 tubes which traverse the parenchyma in all directions. Hanging on to 

 the tubes composing this network are numerous somewhat pear-shaped 

 cells (cf. Fig. 75, A) which if examined in the live condition with a high 

 power of the microscope attract attention by the curious flickering in 

 their interior like that of a candle-flame. These are what are called 

 flame-cells. Each consists of a tube in communication with the general 

 excretory network. The free end of the tube is blocked up by a mass of 

 soft protoplasm containing a nucleus, and bearing a tuft of large cilia 

 or flagella which project from it down the cavity of the tube. It is the 

 movement of these flagella that gives the characteristic flickering appear- 

 ance. The waves of flexure passing down the flagella from base to tip 

 tend to set up a negative pressure within the closed end of the tube 

 which causes the watery fluid from the parenchyma meshwork to be 

 drawn into the tube through the protoplasmic plug and then sent onwards 

 through the tubular network to the exterior. The function of the flame- 

 cells is to keep draining away the excess of fluid in the meshes of the 

 parenchyma, and in some animals in which observation is easier than it 

 is in the fluke it is possible to determine that the activity of the flame-cell 

 is closely related to the pressure of fluid, for diminution of this pressure, 

 for example by letting some of the fluid escape through a small puncture 

 of the body -wall, causes instant cessation of the flagellar movement, 

 which commences again as the pressure is re-established. 



The nervous system as in all internal parasites is very simple a 

 collar with three ganglia round the anterior part of the alimentary canal 

 sending back two long trunks towards the hind end of the body : and 

 there are no special organs of sense. 



The fluke is hermaphrodite and, as so often happens in such cases, 

 the reproductive organs are exceedingly complicated (Fig. 76). They 

 are also of great size, as is again very usual in parasites where there is 

 as a rule enormous wastage through the great majority of young that 

 are produced failing to reach a new host. 



M 



