Chapter 19 



FLATWORMS AND ROUNDWORMS 



(PHYLUM PLATYHELMINTHES and 

 PHYLUM NEMATHELMINTHES) 



Planaria (Dugesia) ; Liver Fluke; Tapeworm; Ascaris 



Planaria (Dugesia) (pla-nar'ia) (Gr. pianos, wandering) and the 

 liver fluke Fasciola hepatica (fas -i' o lah) (L. jasciola, a band) (he 

 pat'ika) (Gr. hepar, liver) belong to the Phylum Platyhelminthes (plat 

 i hel -men' thez) (Gr. platus, flat; helmins, worm). 



PLANARIA (DUGESIA) 



The common, free-living, fresh- water planarian is Dugesia tigrina, for- 

 merly called Planaria maculata. Its upper surface is spotted, brown and 

 white, while the lower is grayish. The body has bilateral symmetry and 

 may be 20 mm. long (Figs. 177 to 179) . 



Integument. — The ectoderm, a thin layer of external, ciliated cells, 

 called the epidermis, secretes mucus which may give protection and 

 diminish friction. Rodlike rhabdites (rab' dite) (Gr. rhabdos, rod) em- 

 bedded in the epidermis are discharged for offensive purposes. The 

 entoderm is a single layer of elongated, epithelial cells and lines all 

 branches of the digestive tract. The middle, cellular mesoderm is com- 

 posed of large amoeboid cells. Hence, planaria is triploblastic, being 

 composed of cellular ectoderm, mesoderm, and entoderm. There is no 

 true body cavity. 



Motion and Locomotion. — The entire surface is covered with hairlike 

 cilia, although they are more numerous on the flat, ventral side. Three 

 sets of muscles, longitudinal, circular, and oblique, in the body wall aid 

 in locomotion. Mucus secreted at the anterior end reduces friction dur- 

 ing locomotion. 



Ingestion and Digestion. — The pharyngeal chamber, with its cylindri- 

 cal, muscular pharynx and mouth at its tip, is located midway between 



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