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EVOLUTION OF ANIMALS 



Part V 



times. They are commonly two to six inches long and all are of leaf-like thin- 

 ness (Fig. 25.10). They live almost entirely on the rocky seashore, gliding over 

 the rocks or swimming by the undulating motions of their fluted bodies. Many 

 are inconspicuous; others are strikingly dappled and striped; all swim with a 

 peculiar grace and rhythm that has made them the "butterflies of the sea," 

 competitors with a group of the snails for that name. 



Class Trematoda 



General Characteristics. Trematodes are called flukes (Anglo-Saxon, flok = 

 flat) because of their flat shape. They are built on the turbellarian plan, but 

 are parasites that have become extremely dependent upon other animals. The 



Fig. 25.9. A cosmopolitan land planarian, 

 Bipalium kewense, sometimes brought to 

 northern greenhouses on tropical plants; 

 also found in Florida, Louisiana, and Cali- 

 fornia. It is nearly a foot long, has an ex- 

 panded head and is marked by long purple 

 to black stripes on a yellowish ground; 4, 

 eye; 5, creeping sole. It moves on a creep- 

 ing sole like the fresh-water planarians, oc- 

 casionally hanging off into the damp air. 

 (Courtesy, Hyman: The Invertebrates, vol. 

 2. New York, McGraw-Hill Book Co., 

 1951.) 



adults cling to their host by one or more suckers, and their bodies are covered 

 with tough cuticle. They have an enormous reproductive capacity and live 

 parts of their life span in alternate hosts. Like other parasites, they lack some 

 of the features that are present in their free-living relatives, external cilia, an 

 epidermis, rhabdites, and eyes. 



Flukes attack large numbers of vertebrates, including domestic animals and 

 man. Their life cycles are complicated and their existence a gamble. Certain 

 trematodes have relatively direct development and one host (Order 

 Monogenea). Most of these are ectoparasites on the gills and skin of fresh- 

 water and marine fishes; some of them live mainly in the urinary bladders of 

 frogs. The fertilized eggs are shed into the water and there develop into ciliated 

 larvae that gradually become like their parents, first in their clinging habits and 

 then in structure including the gradual loss of ciha and of eyes. 



