FLAT-WORMS. 



459 



The Flat- Worms, — Class Platyhelminthes. 



The flat-worms are characterised by the absence of a distinct vascular system, 

 and by the alimentary canal being either absent or with no posterior outlet. A 

 nervous system is developed, consisting either of a network of nerves, or some- 

 times of a distinct brain and lateral chords. The excretory organs are composed 

 of fine tubules opening to the exterior, and the body-cavity is reduced to a set of 

 slits in the tissues. For the most part the sexes are united in one individual. 

 Sometimes a sexual reproduction occurs, accompanied by an alteration of genera- 

 tions. There are three orders of flat-worms, namely, the tape - worms, the 

 trematodes, and the turbellarians. 



cross-bearing nemertine {Polia crucigera) on a coral (uat. size). 



Tape- Worms, — Order Cestoda. 



The members of this extensive group are internal parasitical worms with 

 the body divided into a number of segments. There is no trace of an alimentary 

 canal, nutriment being obtained by the absorption of juices through the entire 

 surface of the body. The head is furnished with suckers, or hooks, or both, by means 

 of which the worm adheres to the walls of the intestine of the host it infests. 

 The nervous system consists of a ganglion in the head, and a cord on each side. 

 As a well-known example, we may take Tamia saginata, one of the human tape- 

 worms ; and since its structure and the phases through which it passes in the 

 course of its development are thoroughly known, a detailed discussion of its 

 characteristics will serve as an introduction to the study of the group. In its 



