CHAPTER I.
SHELLS AND THEIR INMATES.
BEFORE the study of shellfish, or molluscs, was conducted on the scientific principles of the present day, shells were classified as univalves, bivalves, and multivalves. The univalves were shells in one piece, such as the whelk; the bivalves those in two pieces, such as the mussel or oyster; and the multivalves those in more than two pieces, such as barnacles or chitons, barnacles, however, being no longer classed with shells.
The highest of the five types, or natural divisions, of animals are the Vertebrata, the Mollusca, and the Annulosa. The vertebrates usually have vertebrae, or jointed backbones, and from this the highest division takes its name; but the real test is the colour of the blood, which in the vertebrates is always red.
The molluscs have soft bodies and no internal skeleton, but in lieu of this the animal is usually protected by an external shell, harder than the bones of vertebrates. The annulosa, like the molluscs, have soft bodies and no internal skeletons; but the external shell is divided into joints or segments, and is usually softer than the bones of vertebrates.
Fishes belong to the vertebrate division, oysters to the mollusc, and crabs and starfish to the annulosa.
The remaining two of the five divisions are the Caelenterata, in which the general cavity of the body communicates freely with that of the digestive apparatus, and the Protozoa, which includes all animals, such as sponges, etc., not included in the above four divisions.
The shell of an oyster takes the place of the bones of a dog; and although it may seem strange for an animal to have its bones on the outside of its body, it is really no more strange than for a fruit, such as the strawberry or raspberry, to have its seeds on the outside. Lime is the principal ingredient of all bones; and the bones of vertebrate animals contain a large proportion of phosphate of lime, while the shells of molluscs, or shellfish (as they are popularly called), consist almost entirely of carbonate of lime.
