PHYLUM PLATYHELMINTHES 



167 



buccal cavity which it nearly fills; a three-branched enteron or in- 

 testine, which branches immediately from the anterior end of the 

 pharynx into an anterior trunk; and two lateral trunks that turn 

 posteriorly, one along each side of the pharynx, and extend nearly to 

 the posterior end. The pharynx is in the form of a cylindrical fold 

 projecting through the full length of the pharyngeal chamber. It is 

 attached only at its proximal or anterior end and is perfectly free 

 otherwise. When it is extended or protruded through the mouth 

 opening which it fills, it forms a proboscis whose length may be as 

 great as, or greater than, that of the entire body. The trunks of the 

 enteron have many lateral, blind extensions or pockets called diver- 

 ticula which greatly increase the surface exposure of the organ and 

 project among most of the other tissues of the body. The whole 

 arrangement represents a complicated gastrovascular cavity whose 

 wall is endodermal. 



Cilia 



Excrelioiy tubule 



Fig. 83. — Flame cell of planaria. 



The excretory system is new to our study and is composed of a set 

 of tubules which relate themselves to all parts of the body. There 

 are two principal, longitudinal, coiled tubules, one along each side 

 of the bod}', which receive many small branches and open by minute 

 pores located just posterior to the eyespots, and by several other pores 

 along the length. Each of the numerous smaller branch-tubules has 

 at its blind end a flame cell which is hollow and contains a mass of 

 long cilia that are continually beating in a direction toward the 

 tubule, the movements appearing something like a flickering flame. 



