TUBIFEX. 391 



Cestoidea Monozoa. The parasite occupies the coelomic 

 cavity, and has only been found during the summer 

 months, when the reproductive organs are undeveloped. 

 The mature form is to be found in the intestine of certain 

 fishes, and the } r oung in the anterior segments of Tubifex 

 rivulorum, usually in segments 10 to 17. Each one is 

 provided at the anterior end with a characteristic mobile 

 organ capable of being thrown into a series of undulating 

 folds. At the opposite end the cylindrical body is 

 produced into a tail provided with three pairs of hooklets. 



