78 BRACHIOPODA. 



by the whole breadth of the ventral valve, but com- 

 monly by a muscular foot-stalk passing through 

 the perforated beak. In general this stalk is only 

 long enough to allow the dorsal valve room to lie 

 upon or hang against the supporting body ; but in 

 some species, as in the genus Lingula, it is six or 

 seven times as long as the shell. 



Between the bases of the arms is concealed the 

 mouth, which leads to a digestive canal more or 

 less complex, and of varying length. The dis- 

 charging orifice is usually near the perforation. 



The valves, like those of the CONCHIFERA, are 

 lined with the leaves of a fleshy mantle, the mar- 

 gins of which carry, in lieu of flexible tentacles, 

 a fringe of stiff brittle bristles, which project be- 

 yond the edges of the valves, and are probably 

 organs of touch. Besides the office of secreting 

 the shell, the mantle in this Class performs the 

 function of respiration by its internal surface. The 

 mantle-leaves are also permeated by ample blood- 

 vessels, which ramify through them. 



Little is known, as yet, of the nervous system 

 in these animals ; and scarcely more of the phe- 

 nomena of their reproduction. The ovaries in some 

 cases surround the ramifications of the blood- 

 vessels within the mantle, and in others occupy 

 large cavities on each side of the body. Some 

 facts seem to indicate that the young fry are 

 hatched within the valves of the parent ; but what 

 is their first form, and what the extent of their 

 metamorphoses, we are ignorant. 



All the known species of the Class are marine, 

 but comparatively few exist now, the great ma- 

 jority being fossil. Its British representatives are 

 very few, extending only to five species. 



