DIGESTIVE AND VASCULAR SYSTEMS 



415 



enormously in different 

 individuals. The aliment- 

 ary canal near the anus 

 also bears two tuft - like 

 organs, which, however, do 

 not open into the intes- 

 tine, but probably have 

 some function in con- 

 nexion with the fluid in 

 the body-cavity. 



Along the whole course 

 of the alimentary canal 

 there runs a ciliated groove, 

 into which the food does 

 not pass, but the cilia of 

 which probably keep in 

 motion a current of water 

 whose function may be 

 respiratory. 



Vascular System. /-- 

 On the dorsal surface of 

 the anterior end of the 

 alimentary canal lies a j - 

 contractile vessel, usually 

 termed the heart. It is 

 a tube about an inch long, 

 ending blindly behind, but 

 opening in front into a 

 ring - shaped space sur- 

 rounding the mouth and 

 partially enveloping the 

 brain. From this ring-like 

 vessel numerous branches 

 are given off which pass 

 into the fringe round the 

 mouth, and probably the 

 chief function of the heart 

 is by its contraction to 

 force fluid into this fringe, 

 and so to extend it. The heart contains a 



9- 



corpusculated fluid. 



