160 MANUAL OF NATURAL HISTORY. 



BKACHIOPODS. 



The Brachiopods are headless Mollusks like the 

 Conchifera, and the foot is also wanting ; they are, 

 moreover, destitute of true gills, and appear to re- 

 spire by means of their mantle. Their bodies are 

 protected by two unequal, symmetrical, valves united 

 by a hinge without a ligament ; the under or ven- 

 tral valve is sometimes attached, and the upper or 

 dorsal is frequently perforated near the beak for the 

 passage of a tendon, by means of which the animal 

 is anchored to sub-marine bodies. The mantle-mar- 

 gins are disunited, but the great distinguishing fea- 

 ture of these remarkable creatures is the existence 

 of two, strangely contorted or spiral, bony arms 

 covered with a ciliated membrane, and arising from 

 each side of the mouth. The ends of the spiral 

 arms, in some genera, are endowed with voluntary 

 motion, the movement being effected by the injec- 

 tion of a fluid into the hollow spiral tube by which 

 the coils are separated. The peculiar bony apophy- 

 sary skeleton exhibits various modifications ; two 

 thin processes usually proceed from near the hinge 

 of the upper perforated valve, and form loops or 

 simple apophyses ; sometimes there is a median per- 

 pendicular lamella between them, and sometimes 

 they develope other processes, and constitute a very 

 complicated apparatus. Their nervous system is 

 composed of several ganglia surrounding the oeso- 



