370 



FRESH-WATER BIOLOGY 





of the body or close to it and in the anterior or oral sucker is com- 

 monly shaped like a tuning fork (triclad). More rarely it is rod- 

 like (rhabdocoel), or branching (dendritic). A sphincter, the 

 pharynx, is ordinarily found on the esophagus 

 and the true digestive region consists of the 

 two branches, the ceca or crura, which vary 

 greatly in length. 



The excretory system (Fig. 651) usually 

 . opens at the opposite end of the body, and 

 is I-, Y-, or U-shaped. The main branches 

 are distinct, containing in life a clear fluid 

 with a slightly yellowish or bluish tinge. 

 The finer branches can be traced only with 

 difficulty. They terminate in the essential 

 . excretory elements known as " flame cells" 

 Af- which ma y be distinguished readily only in 

 the living animals under high magnification. 

 In the larger tubes one finds commonly highly refractive granules 

 of excretory material. 



Of the nervous system one can usually see irregular masses 

 (ganglia) right and left of the alimentary canal, near its anterior 

 end. They are joined to form a sort of collar around the esopha- 

 gus, and from them nerves pass anteriad and posteriad throughout 

 the body. Further details of structure can be followed only by 

 special methods and in well-preserved specimens. 



Special sense organs are not common. A few of the ectopara- 

 sitic trematodes, which are rare in fresh water, have pigmented 

 eye-spots near the brain, and the free-swimming stages of endo- 

 parasites show similar structures which with rare exceptions are 

 wanting in the adult internal parasites. 



The reproductive system is the most conspicuous part of the 

 worm but is exceedingly complicated and often difficult to follow. 

 Yet it is the most important feature in the classification of the 

 group. Most flukes are hermaphroditic, and contain complete 

 organs of both sexes. The arrangement of these organs in a simple, 

 typical case is given in the accompanying diagram (Fig. 652). 

 In many species an enormous accumulation of eggs in the uterus 



