CHAPTER 12 



4l\o*t 



Phylum 



Platyhelminthes. 



Simple Organ-System 



Animals 



HE Platyhelminthes are much flattened 

 dorsoventrally and hence are known as flat- 

 worms. Among them are both free-living 

 and parasitic species; the former live prin- 

 cipally in fresh or salt water; the latter are 

 mostly endoparasitic. The parasitic flat- 

 worms are known as flukes or trematodes, 

 and tapeworms or cestodes. They arc widely 

 distributed among human beings and other 

 vertebrates; they are often pathogenic, and 

 sometimes bring about the death of the 

 host. Free-living flatworms of North Amer- 

 ica live in springs, ponds, and streams, or in 

 bodies of salt water. 



Flatworms exhibit many advances over 

 the coelenterates and ctenophores. They are 

 definitely bilaterally symmetrical, a charac- 

 teristic common to most of the animals 

 above them in the scale of life. This type of 

 symmetry is correlated with various modifi- 

 cations both in structure and physiolog). 

 The flatworms possess a third embryonic 

 tissue; hence their structures are derived 

 from ectoderm, endoderm, and mesoderm 

 (Fig. 92). The mesoderm gives rise to all 

 tissue between the epidermis and intestine, 

 except the nervous tissue. 



Like most of the animal characters, the 

 mesoderm has been foreshadowed in the 

 more primitive animal groups. Its early be- 

 ginnings are probably represented by some 

 of the mesenchyme cells of the coelen- 

 terates. However, mesenchyme is not gen- 

 erally considered true mesoderm until, as 

 in the flatworms and other more complex 

 animals, it is more massive and gives rise to 

 definite structures such as muscles. The 

 Platyhelminthes is the lowest phylum of 

 animals built on an organ-system level of 

 complexity. It is also the first phylum in 

 which there is a distinct head with sense 

 organs and central nervous system. The 

 commonest free-living species of the flat- 

 worms are called planarians and for this rea- 

 son a planarian has been chosen for special 

 description. 



Two entire classes of flatworms are para- 

 sites, some of which, such as the tapeworms, 



133 



