CHAPTER V 



Platyhelminthes 



The Platyhelminthes (Gr. plains, broad; helminthus, an intes- 

 tinal worm) are soft bodied, bilaterally symmetrical and dorsi- 

 ventrally flattened worms lacking the true segmentation character- 

 istic of the earthworm. The majority of the Turbellaria are free 

 living, the Trematodes are all ectoparasites or endoparasites, and 

 the Cestodes are all endoparasitic. Some parasitic flatworms re- 

 quire several hosts in order to complete their life history. 



Platyhelminthes have two embryonic layers, the ectoderm and 

 endoderm. They differ from higher worms in that they have no 

 coelom. A packing tissue, the mesenchyme, forms a compact mass 

 oi pa7-enchyma (connective tissue) occupying the space between the 

 organs and the body wall. The majority are hermaphroditic, i.e., 

 with the gonads of both sexes in one individual. The digestive 

 tract, when present, is a coelenteron with no anal opening. 



Classification 



Class 1. Turbellaria (Lat. turbo, I disturb), Planaria. 



Class 2. Trematoda (Gr. trema, a pore; eidos, resemblance), 



Distomes, liver flukes. 

 Class 3. Cestoda (Gr. kestos, girdle; eidos^ resemblance), tape 



worms. 



Characteristics 



I. Flattened dorsiventrally. 

 1. Bilaterally symmetrical. 



3. Do not bud to form a colony, but do form linear chains (tape 



worm). 



4. Lack a coelom, the spaces between the organs and body wall 



being occupied by connective tissue called parenchyma. 



5. Excretory system of paired branched proto-nephridia or flame 



cells, connected in a water vascular system. 



6. Nervous system consists of a supra-esophageal ganglion with main 



ventral nerve trunks. 



7. Have two embryonic layers. 



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