CHAPTER XXII. 

 BRANCH PLATYHELMINTHES. 



The Flatworms. 



[THE word " worm " is used in a very loose and indefi- 

 nite way. In popular language it is applied indiscrimi- 

 nately to the legless earthworm, the insect larva with 

 segmented appendages (caterpillar), and to the elongated 

 mollusk styled the shipworm. All that is required to 

 merit the title is a soft, elongated, bilaterally symmetrical 

 body. This superficial view makes no distinction as to 

 whether the body is segmented or unsegmented, whether 

 appendages are present or absent, whether the form is 

 that of an adult or only in the larval stage, and asks no 

 questions as to internal structure. The old group of 

 " Worms " had no fundamental unity in plan of structure ; 

 in fact, they had nothing in common except a general simi- 

 larity in form. Hence it is natural that an increasing 

 knowledge should break up the old branch " Vermes." 

 In its stead we find what were formerly reckoned as 

 classes of the branch elevated to the rank of branches, as 

 follows : Platyhelminthes, the flatworms ; Nemathelmin- 

 thes, the roundworms ; Trochelminthes, the rotifers or 

 Wheel Animalcules ; the Molluscoida ; and the Annulata, 

 the segmented or ringed worms, such as the earthworm.] 



The Platyhelminthes. As the name indicates, the body 

 is flattened. They are bilaterally symmetrical, and with- 

 out skeleton or body cavity. There is no system of blood- 

 tubes. The body has three embryonic layers, ectoderm, 



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