THE TEREBRATULA. 35 



In one species of terebratula, these arms are deve- 

 loped to a most extraordinary degree ; when withdrawn 

 and at rest, they are contracted into six or seven spiral 

 folds, and when protruded, are at least twice as long as 

 the shell in which the animal is inclosed. They are 

 hollow, and contain a fluid, which, being acted upon 

 by muscles spirally disposed round the canal, is forcibly 

 driven upwards to the extremity of each arm, and by 

 this singular method they are expanded and protruded. 

 It is to Professor Owen that the discovery of this 

 remarkable apparatus, so simple and yet so efficient, 

 is due. Every species of terebratula, however, has not 

 the arms thus free and lengthened : in terehraUda 

 chilensis they are attached throughout their length 

 to certain thin semicircular elevations on the inside 

 of one of the valves, and their movements are con- 

 sequently limited. 



In all, differing as the arms do in development and 

 liberty, they are the sole agents by which food is pro- 

 cured ; and without them, the animal, fixed and inca- 

 pable of seeing, pursuing, or seizing its prey, would 

 necessarily perish. Furnished with cilia, or fringe- 

 like filaments of a horny texture, and ever in motion, 

 they produce a continuous current of water revolv- 

 ing round the mouth, a simple orifice, which opens 



