76 BEACHIOPODA. 



hinge-line is produced in a straight line to such 

 an extent as to make the valves more or less semi- 

 circular ; occasionally the outline is somewhat tri- 

 angular, and not rarely two-lobed. 



The shell in most of the genera is perforated by 

 tubular canals passing directly from one surface to 

 the other, and opening internally by minute orifices, 

 externally by trumpet-shaped, or sometimes, as in 

 Crania, by many-branched mouths. As to its 

 structure, it is composed of flattened prisms, ar- 

 ranged obliquely to the thickness of the valve. 

 A few genera are composed almost wholly of a 

 horny animal substance ; but in general the earthy 

 element predominates even to a greater degree 

 than in the CONCHIFERA. 



The term BRACHIOPODA, signifying " arm- 

 footed," is now too generally adopted to be con- 

 veniently changed, but the notion conveyed by it 

 is only partially true. It was intended to express 

 the most remarkable character of these animals, 

 the presence of a pair of arms, often of great 

 length, rolled up in a spiral form, which were 

 believed by Cuvier to replace the foot in other 

 bivalves. They are, however, now understood to 

 be the palps immensely prolonged. 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 pro- 

 pelled from the base to the extremity, thereby 

 unrolling, as he believes, the spiral coils. Whether 

 this process of expansion and protrusion is the 

 means by which the animal secures its food, or 

 whether it really takes place at all in the natural 

 * Trans. Zool. Soc. vol. i. (1835). 



