ARM-FOOTED MOLLUSKS. 309 
CLASS V. BRACHIOPODA. 
(Arm-footed Mollusks.) 
THE animals bearing the above title are inhabit- 
ants of bivalve shells, one valve of which is fre- 
quently perforated, to give exit to a fleshy peduncle 
or stem, by which the animal is affixed to the rocks. 
When we open the two valves of the shell, we tind 
each of them lined with a broad membrane, very 
thin, delicate, and semi-transparent, which together 
constitute the mantle. The edges of these mem- 
branes are thickened, and fringed with organs, 
already several times mentioned in these pages, and 
which we shall see assuming great importance as 
we investigate the economy of the lower tribes of 
animals. ‘hese organs are called cilia, and consist 
of very subtile and microscopically minute hairs, 
arranged in close series, and capable of moving in 
unison, with vibratile waves, and of thus producing 
rapid currents in the water. 
At the bottom of the cleft formed by the two 
leaves of the mantle, the mouth is placed, on each 
side of which is a long fleshy process, fringed all 
along one side with delicate hairs. In some species, 
these arms (which give name to the Class) are enor- 
mously developed ; they are free for their whole 
length, and are capable, at the will of the animal, 
of being coiled up in many spiral folds, or of being 
protruded from the shell to a distance equal to 
thrice its length. The mechanism by which they 
