CHAPTEK I 



.TURBELLARIA 



INTRODUCTION : DESCRIPTION OF THE POLYCLAD LEPTOPLANA 



TREMELLARIS APPEARANCE HABITS STRUCTURE : POLY- 



CLADIDA CLASSIFICATION HABITS ANATOMY DEVELOP- 

 MENT : TRICLADIDA OCCURRENCE STRUCTURE CLASSIFICA- 

 TION : RHABDOCOELIDA OCCURRENCE HABITS REPRODUC- 

 TION CLASSIFICATION. 



The Platyhelminthes, or Flat Worms, form a natural assemblage 

 of animals, the members of which, however widely they may 

 differ in appearance, habits, or life -history, exhibit a funda- 

 mental similarity of organisation which justifies their separation 

 from other classes of worms, and their union into a distinct 

 phylum. Excluding the leeches (Hirudinea), and the long sea- 

 worms (Nemertinea) which, though formerly included, are now 

 treated independently the Platyhelminthes may be divided into 

 three branches: (1) Turbellaria (including the Planarians), (2) 

 Trematoda (including the liver-flukes), and (3) Cestoda (tape- 

 worms). The Mesozoa will be treated as an appendix to the 

 Platyhelminthes. 



The Turbellaria were so called by Ehrenberg 1 (1831) on 

 account of the cilia or vibratile processes with which these 

 aquatic animals are covered, causing by their incessant action, 

 tiny currents (" turbellae," disturbances) in the surrounding 

 water. The ciliary covering distinguishes this free-living group 

 from the parasitic Trematodes and Cestodes, some of which 

 possess such an investment, but only during their early free 

 1 Hemprich and Ehrenberg, Symbolae physicae, Berlin, fol. 1831. 



