98 



ZOOLOGY 



ously regarded as excretory in nature, and as material for 

 counteracting the acidity of the digestive fluids of their host. 



The excretory system consists of an annular ring in the 

 head, from which four ducts corresponding in position with 

 the suckers pass backward. Two of these soon disappear, and 

 the other t-v^o pass down one on each side of the proglottides, 

 just inside the longitudinal muscle layer (Fig. 67). These 



FlQ. 67. — Transverse section through a mature proglottis of Taenia. 



two lateral ducts coalesce at the posterior end of the last 

 proglottis, and open there by a common vesicle. A transverse 

 vessel at the posterior end of each proglottis serves to place the 

 two longitudinal ducts in communication. The main ducts are 

 provided with valves, which only permit the flow of fluid 

 towards .the external opening. A series of secondfiry ducts 

 arise from these main ones and ramify all over the body. The 

 secondary ducts give of!' still finer tubules, each of which ends 

 in a flame cell. The cilium of this cell hangs down into the 

 lumen of the tubule, which is here slightly enlarged and 

 funnel-shaped. Some observers maintain that this funnel- 

 shaped end of the tubule opens by a pore into the splits in the 

 parenchyma which represent the coelom, but this is a disputed 

 point. 



There is a central nervous system in the head. This gives 



