636 - Heredify and Evolution 



large majority of flatworms are parasites, in 

 man and many other animals. Moreover 

 there arc many pathogenic species. In fact, 

 intestinal flukes, liver flukes (Fig. 32-13), 

 blood flukes, and tapeworms (Fig. 32-14) may 

 cause serious illnesses in ihe various host 

 organisms which (hey infect. 



Characteristics of the Phylum. Some very 

 important characteristics of higher animals 

 generally appear to have originated ai the 

 evolutionary level ol ihe flatworms. These 

 include: (1) bilateral symmetry; (2) a tend- 

 ency toward cephalization (differentiation of 

 a definite head region); (3) a clear-cut triplo- 



blastic organization, in which an unmistak- 

 able mesoderm is represented; and (4) the 

 development of well-defined organs and 

 organ systems. However, the Platyhelminthes 

 also have many primitive features: (1) When 

 present, the enteron is of the saccular (rather 

 than the tubular) type. (2) Food vacuoles fre- 

 quently are formed by the cells lining the 

 enteron. (3) No true body cavity, or coelom, 

 is present. (4) No definite skeletal respira- 

 tory, or circulatory organs occur. (5) Locomo- 

 tion (in free-living forms) is by means of 

 (ilia. 



Bilateral symmetry probably originated 



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Fig. 32-13. Stained specimen of a parasitic flatworm, the liver fluke {Clonorchis sinesis). This adult trema- 

 tode infests the human gall bladder. Other stages in the life cycle are shown in Figure 32-16. {Copyright, 

 General Biological Supply House, Inc.) 



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Fig. 32-14. Scolex ("head") and young proglottids (sections) of tapeworm from human intestine, slightly 

 enlarged. Note the ring of hooks (extreme left) and the suckers (immediately to right of hooks) with which 

 the worm fastens itself to the intestinal wall. (Copyright, General Biological Supply House, Inc.) 



