25 



Flatworms — Vanguard oi 

 tne Hi^ner Animals 



There is a vast difference in the relative speed of a flatworm and a race 

 horse, yet bilateral symmetry, always the partner of speed, had its beginning in 

 the flatworms. They were the first animals in the evolutionary procession to 

 firmly establish the likeness of two sides of the body feebly suggested in the 

 ctenophores (Fig. 24.1). Other features of higher animals begun in the free- 

 living flatworms are a definite head, a centralized nervous system, the meso- 

 derm or middle layer of body cells, and complex reproductive organs. 



Along with their advances in build and behavior flatworms are strikingly 

 primitive. In the majority of the free-living species the digestive tract has but 

 one opening, the mouth. Instead of being located in the head, it is in the center 

 of the underside of the body suggesting the wheel-like symmetry of the jelly- 

 fishes, in which the mouth takes the place of the hub (Figs. 25.1). 



Classes of Flatworms. There are three main classes in the Phylum Platy- 

 helminthes and each one has its particular successes: 1, Turbellaria are, for 

 the most part, free living, have a digestive cavity and are covered with cilia; 

 2, Trematoda have a digestive cavity, cuticle covering the body but no cilia, 

 and are parasitic; 3, Cestoda lack an enteron and have a cuticle, but no cilia. 



Turbellaria — Planarians and Others. The power to regrow lost parts 

 permits planaria to survive and even to multiply after injury. Three planarians 

 may live and flourish because one was cut into three pieces. All turbellarians 

 are aquatic. A considerable number live in fresh water and a few on moist soil. 

 Most of them are marine. Some are marked with striking patterns in yellows, 

 reds, and black and white and all are graceful swimmers and gliders. They are 

 named for the turbulent movements of their abundant cilia. 



Trematoda — Liver Flukes and Others. All trematodes have great 

 capacity to multiply. They are parasites, with a wide distribution assured them 



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