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EVOLUTION OF ANIMALS 



Part V 



bryozoans has replaced their other name Polyzoa. All are aquatic and upwards 

 of nearly 3000 species are marine; only about 35 live in fresh water. The 

 marine species are widely distributed in coastal waters, between the tide lines. 

 They grow on rocks and seaweeds, easy to see — but not to distinguish as 

 animals. Most of the colonies seem to be only white, yellow, or brown patches 

 of crust on the damp stones and seaweeds (Fig. 27.11). Other colonies might 

 be delicate branching seaweeds, two to four inches high, rooted to rocks and 

 kelp. The common fresh-water Plumatella resembles a dark vine with white- 



FiG. 27.10. A typical gastrotrich, Chaetonotus. They are 

 many-celled fresh-water animals of microscopic size, like 

 their neighbor rotifers. They are so abundant, widely dis- 

 tributed, and striking in appearance that they demand at- 

 tention even among hordes of other minute animals. (Cour- 

 tesy, Robert W. Pennak, Fresh-Water Invertebrates of the 

 United States. Copyright 1953, The Ronald Press Company.) 



tipped branches, actually the folded tentacles of the animals. Colonies of the 

 fresh-water Pectinatella magnifica live on the surface of great blobs of jelly 

 which they secrete about submerged stems. Algae invade the jelly and the 

 whole object might be a green pineapple floating in the midsummer pond. If 

 they are taken from the water none of these colonies gives the slightest sign 

 of life, but immersed in it, each animal puts forth its exquisite plumy crest on 

 the regular business of gathering food. 



Structures and Functions. The common bryozoan Bugula grows on the east- 

 ern and western coasts of North America in tufts two or three inches long, 

 attached to seaweed. Although they are members of a colony, each individual 

 lives independently of its neighbors (Fig. 27.12). In this type of bryozoan, 

 each animal is protected within a homy tube; in others, every animal is in a 

 limy cup or surrounded by jelly. The characteristic and, under a lens, con- 

 spicuous feature of each animal is the lophophore which bears hollow flexible 

 tentacles astir with cilia that draw diatoms and protozoans into the mouth, 

 whence they are passed along the digestive canal by more cilia. 



