PSEUDOCOELOMATE AND MINOR EUCOELOMATE PHYLA 



generally short, solid, and fibrous; no anus is present; and a calcareous 

 "shelly loop" supports the lophophore. Fundamental difTerences also appear 

 in the embryonic development of the two groups. 



The brachiopods are an ancient group. Not more than 260 species are 

 now known to be living; but more than 7000 species have been recorded as 

 fossils. The existing genus Lingula is represented by species in the earliest 

 fossil-bearing rocks of the Cambrian period (roughly half a billion years old) 

 and in later deposits. Our present species have apparently undergone very 

 little modification, and this is probably the oldest known genus of animals 

 represented among our contemporary fauna. 



The genus Terebratnlina represents the great majority of modern brachi- 

 opods. The calcareous valves of the shell are both opened and closed by 

 specific groups of muscles, and other groups of muscles adjust the position 

 of the animal in relation to its stalk. The upper (dorsal?) valve is the smaller 



Fig. 12.14. Representative Brachiopoda. A, Lingula, shown in its burrow in a sandy sea 

 bottom; Lingula is a modern example of a type of organization that has persisted with httle 

 change through about half a billion years of the earth's history. B, Terebratnlina, repre- 

 senting a more progressive type which has developed heavier calcareous valves, hinged to- 

 gether, and a calcareous support for the lophophore. This tentacle-bearing feeding structure 

 can be seen in the individual with partly opened valves. {A, photograph by Eugene S. Clark, Jr.; 

 B, photograph by George Lower. ) 



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