CHAPTER 9 



THE THREE-LAYERED ANIMALS 



Quite different from the symmetrical 

 beauty of hydra, the jellyfishes, and the sea 

 anemones, are the drab representatives of 

 the next group of animals, the flat worms. 

 Their flattened and elongated bodies ac- 

 count for the name of the phylum to which 

 they belong, Platyhelminthes (from the 

 Greek, platij — flat). Just as the two-layered 

 animals showed distinct advantages over 

 the unicellular animals, so the flatworms 

 demonstrate a higher form of life than was 

 observed among the coelenterates. 



The most important single morphological 

 structure acquired by this group is a third 

 body layer, the mesoderm. It is only with 

 the advent of this additional layer that ani- 

 mals were able to reach higher levels of 

 complexity. Most of the intricate and mas- 

 sive structures of not only platyhelminthes 

 but all higher animals have been derived 

 from this layer. We see a forerunner of the 



mesoderm in the mesogloea found in the 

 coelenterates, although in this group it 

 never became a distinct layer. Once the 

 mesoderm was established in the flatworms 

 it was retained by all subsequent groups of 

 animals. 



As a result of the introduction of a third 

 body layer other modifications were possi- 

 ble. One of the most obvious of these was 

 a change from radial to bilateral symmetry. 

 This meant the acquisition of head and tail 

 ends, dorsal and ventral sides, and left and 

 right sides. Localization of the sensory sys- 

 tem in the head region was initiated in 

 these forms, signifying a definite step to- 

 ward centralization of the nervous system. 

 Moreover, the animal now moved in one 

 direction to seek food rather than acquir- 

 ing its meal in a passive manner as was true 

 of the coelenterates. All of these changes 

 resulted in a much more complex animal, 



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