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ANIMAL BIOLOGY 



form branching colonies which secrete a skeleton of a 

 tough, horny substance resembling chitin. These include 

 the sea-mats, sea-fans and black corals, etc., many of 

 which are commonly mistaken for seaweed. The grace- 

 ful form and vivid colors of many of the colonies of coral 



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FIG. 115. The common bath sponge, i, shows the sponge with parts 

 cut away to show the channels through which water flows in and out, 

 the arrows indicating the directions of the flow; 2, a part of the surface 

 highly magnified; 3, a section through the sponge showing the horny fibers, 

 /, the incurrent orifices, o, and the chambers lined by ciliated cells; 4, 

 ciliated chambers enlarged, a single collared cell at c. (From a Pfurt- 

 scheller wall chart.) 



polyps often give to coral reefs the appearance of beauti- 

 ful submarine gardens. Here thrive numerous other ani- 

 mals which seek the shelter afforded by masses of coral 



