CHAPTER I. 
SHELLS AND THEIR INMATES. 
Berore 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 verte- 
brates 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 mol- 
lusc, 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. 
