Fig. 11.1. A typical fresh-water triclad tur- 

 bellarian, Dugesia ligrina. Note the three 

 chief branches of the gut, each with numerous 

 branching subdivisions. (Photograph cour- 

 tesy General Biological Supply House, Inc.) 



Cestoda, tapeworms. The flukes and tapeworms are widely known, at least 

 by name, but the turbellarians, although most representative of the Platy- 

 helminthes, are not familiar animals. 



Most Turbellaria are small and inconspicuous. They live in the shallow 

 waters of the ocean and in fresh water; some tropical species are terrestrial 

 but are found only in humid places. A few turbellarians are parasitic upon 

 the outer surfaces of other animals. In making comparisons between flat- 

 worms and other phyla it is necessary to use the Turbellaria alone, since the 

 Trematoda and Cestoda are much specialized in adaptation to their parasitic 

 existence. When the Turbellaria are thus used to represent the Platy- 

 helminthes, the members of this phylum are seen to exhibit certain primitive 

 features, such as a single opening to the digestive tract, and also more 

 specialized features, such as their bilateral symmetry. They are, in fact, the 

 simplest members of the Bilateria. The Trematoda and Cestoda are im- 

 portant because they present excellent examples of the structural modifica- 

 tions characteristic of parasites, and because they include many parasites 

 dangerous to man and to domesticated animals. 



THE CLASS TURBELLARIA 



Five orders of Turbellaria are generally recognized, ranging from very 

 simple to rather complex animals. These orders are distinguished partly by 

 the nature and shape of the digestive tract and are descriptively named; thus 



315 



