,5 



1 70 UNSEGMENTED "WORMS." 



Platyhelminthes, and the mesoderm is sometimes not more 

 definite than in Ctenophora. 



PHYLUM PLATYHELMINTHES. 



The Platyhelminthes or flat-worms include three chief classes 

 -Turbellarians, Trematodes, and Cestodes which form a re- 

 lated series. The body is flattened from above downwards ; 

 the mesoderm forms a compact mass of cells or parenchyma 

 without a definite co?lom ; there is the beginning 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 

 to notice that the Turbellarians and Temnocephalids are 

 free-living, except in the case of a few marine Turbellarians 

 which have taken to parasitism ; that the Trematodes are 

 all parasitic, either external hangers-on (ectoparasites) or 

 internal boarders (endoparasites) ; and that the Cestodes 

 are altogether endoparasitic. It is probable that the flukes 

 and tape-worms arose from Turbellarian-like ancestors which 

 adopted parasitic habits. Attention must be directed to 

 the flame-cells which are characteristic of Platyhelminthes. 

 Each terminal twig of a branch of an excretory canal 

 leads into a large hollow cell, from the base of which 

 a bunch of cilia - - with rapid movements suggesting a 

 flickering flame - - projects into the cavity towards the 

 lumen of the twig. 



Class TURBELLARIA. Planarians, etc. 



Turbellarians are unsegmented "worms" usually leaf-like, 

 living in fresh, brackish, or salt water, or in moist earth. 

 Almost all are carnivorous, a few are parasitic. They re- 

 present the beginning of defi?iite bilateral symmetry. 



The ectoderm is ciliated, often glandular, often with peculiar 

 rod-like bodies (rhabdites] which may be discharged on irrita- 



