158 AN INTRODUCTION TO ZOOLOGY 



FLAT WORMS IN GENERAL 



Flat worms are usually separated into three classes. 



Class i. Turbellaria. Flat worms with ciliated epidermis 

 and digestive cavity; mostly non-parasitic. Example, Planaria. 



Class 2. Trematoda. Flat worms without ciliated epi- 

 dermis; digestive apparatus well developed; ecto- or endo- 

 parasitic. Example, liver fluke of sheep. 



Class 3. Cestoda. Flat worms without ciliated epidermis 

 and digestive apparatus; endoparasitic. Example, tapeworm. 



Planaria shows the principal features characteristic of flat 

 worms; but there must necessarily be wide diversity in structure 

 among the members of a phylum composed of both free-living 

 and parasitic forms. The free-living flat worms, such as Planaria, 

 probably are more nearly like the ancestors of this phylum than 

 the parasitic species, since the latter have undoubtedly become 

 degenerate with respect to certain structures, and more special- 

 ized with respect to others, because of their modified habits of 

 life. From a study of Planaria, therefore, we can gain some idea 

 of what kind of an animal gave rise to the flat worms. 



In the first place definite bilateral symmetry is exhibited here. 

 Flat worms thus show an advance in this respect over the more 

 simple radial symmetry of Ccelenterates. A second point to be 

 noted is the presence of a distinct mesoderm between the ecto- 

 derm and entoderm. This mesoderm consists of muscle cells 

 and connective tissue. As in the Coelenterates, however, there 

 is but one body cavity, represented by the digestive system, 

 though the genital sacs may represent a second cavity, known 

 as the ccelom, which is well developed in more complex 

 animals. 



The digestive apparatus is not a simple blind sac, as in Hydra, 

 but consists of several large branches, each with many smaller 

 side pouches entering it, the whole being modified to transport 

 nutriment to all parts of the body, a circulatory system being 

 lacking. An anus, however, is absent, the ingestion of food and 



