ZOOLOGY. 



THE INVERTEBRATA. 



PHYLUM PLATYHELMINTHES (FLAT WORMS). 



State the principal distinctive features of the Platyhelminthes, 



The body, flattened dorso-ventrally, is usually either 

 leaf-like or tape-like ; in some instances it approaches a 

 cylindrical shape. Between the ectoderm and the endoderm 

 there is a mesenchyme tissue, the parenchyma, certain 

 cells of which have muscular processes and form strands- 

 of unstriped muscle. There is no coelomic body- cavity. 



The alimentary canal, when present, has only one open- 

 ing, the mouth. The central nervous system consists of 

 two united ganglia (cerebral ganglia) formed from the 

 massing together of nerve cells, and giving off lateral 

 nerves. There is a definite and ramifying excretory system 

 (flame-cells with tubules leading into larger canals) for 

 removing the body wastes. The reproductive organs com- 

 municate with the exterior by genital ducts. There are 

 accessory yolk and shell glands for providing the eggs with 

 nutrient food- yolk and with tough resistant shells. Nearly 

 all the Platyhelminthes are hermaphrodite. Reproduction 

 is sexual and asexual (by budding or by segmentation) ; 

 both modes occur in the life- history of certain species. 



Many Platyhelminthes are parasites in or on other animals ; 

 but there are free-living forms (e.g., Planarians). 



Describe the Excretory System of the Platyhelminthes. 

 What is a Flame- Cell? 



Certain cells grow from the ectoderm into the paren- 

 chyma ; these have tubular processes and form a ramifying 



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