260 MOLLUSCA. 



CLASS V. BRACHIOPODA.* 



A GROUP very limited in number, comprising 

 animals enclosed within immoveable bivalve shells, 

 generally affixed by a fleshy foot-stalk to the rocks. 

 They are provided with two long spiral arms, which 

 can be unfolded and drawn back, producing, by 

 means of the cilia with which they are studded, 

 powerful currents in the water. In Terebratula^ 

 Psittacea, in which these organs are of great length, 

 Prof. Owen describes the mechanism by which they 

 are unfolded, as very simple and beautiful. The 

 stem of each arm is tubular, and contains a fluid, 

 which, being acted on by the muscles spirally dis- 

 posed that form the tube, is forcibly driven towards 

 the extremity, and the organ is thus unrolled and 

 projected. But the most remarkable circumstance 

 in the structure of these animals is, that the whole 

 mantle forms a respiratory organ, the blood being 

 conveyed to all its parts by innumerable ramifica- 

 tions ; while the cilia with which the whole surface 

 is covered, by their incessant motion, bring ever 

 fresh currents of water to be respired. 



* Bgax'iuv, brachion, an arm, and vous, pous, a foot. 



t Terelratusj bored ; one of the valves being perforated. 



