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, etc. 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 



