330 NATURAL HISTORY. 
the only locomotive organ that the animal has, serves, in 
different species, a variety of purposes, sometimes ena- 
bling the animal to leap, sometimes being used to bore 
into sand or mud, and sometimes only serving to fix the 
animal to some solid support. In some there proceed 
from this foot a band of hair-like filaments, called the 
byssus. While fastened to some object by these fila- 
ments, the animal may have some considerable motion 
within certain limits. The gills have two respiratory or 
breathing tubes connected with them, by one of which 
the water passes into the gills, and by the other passes 
out. The water is made thus to go in and out by fine 
cilia in the gills and on the surface of these tubes, which 
keep up a constant waving or fanning motion. There 
are certain nerves, you see, branching about, and they 
are connected with two pairs of ganglia, or little brains. 
The nervous system is very limited, for the animal has 
little need of either thinking, feeling, or motion. 
567. The lateral symmetry, so thoroughly observed in 
the construction of the Vertebrates and the Articulates, 
which was forsaken to some extent in the Cephalous Mol- 
lusks, is in the Acephalous entirely given up. In them 
there are no two corresponding halves of the body. 
568. The Conchifera we divide into two sections—the 
first including those that have not siphons, and the sec- 
ond those that have them. To the first section belong 
the Oysters, Scallops, Pearl Oysters, ete. The shell of 
the Oyster has two unequal valves. One of these bulges 
out more than the other, and it is by this that it is fast- 
ened to rocks, or pieces of wood, or to other Oysters. | 
The structure of this animal is even more simple than 
that sketched in Fig. 261. It has no foot; for, as it is 
fixed by its shell in one spot, it needs none. Oysters are 
very prolific animals, forming immense banks, and thus 
providing quite largely for the sustenance of man. ‘ But 
man,” says Carpenter, “is by no means the only enemy 
to the Oyster. Its body serves as food to many marine 
