FLAT-WORMS 



211 



Phylum Platyhelminthes 



The Platyhelminthes or flat-worms include three chief 

 classes — Turhellarians^ Trematodes, and Cestodes — which 

 form a related series. The body is flattened from above 

 downwards ; the mesoderm forms a compact mass of cells 

 or parenchyma without a definite coelom ; there is the be- 



FiG. 114. 



A. A minute portion of the branched excretory system of a Platyhehninth, 



showing longitudinal duct (I.), with cilia (C.), its branches (IL and IIL), 

 and the terminal flame-cells (IV.). 



B. One of the characteristic hollow flame-cells, leading into the excretory 



tubule (i), showing the long cilia (2), the excretory globules (3), the 

 nucleus (4), and pseudopodia-like processes (5) passing among adjacent 

 cells. 



ginning of a head-brain ; the excretory system consists of a 

 pair of lateral canals, giving off many branches, whose twigs 

 end in peculiar " flame-cells " ; almost all are hermaphrodite. 

 There is no doubt that the three classes, Turbellarians or 

 Planarians, Trematodes or Flukes, and Cestodes or Tape- 

 worms, are related to one another. A fourth class of 

 Temnocephalids must also be admitted. It is interesting 



