MOLLUSCA. 



419 



having the prismatic formation, banded outside by two 

 rings of true pearly matter nacre, better known as 

 " mother-of-pearl." 



Brachiopoda, "Lamp-shells," or as the name literally 

 signifies, arm-footed, was intended to express a most re- 

 markable characteristic of these animals, the presence of 

 a pair of arms, often of great length, rolled up in a spiral 

 form, and believed by Cuvier to replace the foot in other 

 bivalves. Professor Owen has shown that these organs 

 are tubes closed at each end, and contain a fluid, which 

 by the contraction of the circular muscular fibres of 

 which the walls of the tube are composed, is propelled 



Fig. 209. 



1, A transverse section of a small Pearl from a species of Mytilus. 2. Hori- 

 2ontal section of same Pearl magnified 250 diameters, to show prismatic 

 structure, and transverse striae. 



from the base to the extremity, thereby unrolling, as he 

 believes, the spiral coils. One side of each arm is fringed 

 with a vast number of long filaments, these are ciliated. 

 The shell is opened by a peculiar process, which has given 

 to the Terebratula the name of coach-spring shell. In the 

 shell there are minute openings surrounded by a series of 



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