th 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.

When scientists began more carefully to examine the structure of
shellfish, they found that those similarly constructed had shells with
certain marked peculiarities. The days of conchology were then doomed;
and the study of the mollusc, or malacology, took its place.

Besides those necessary for digesting food, most shellfish have organs
equivalent to those of vertebrate animals, such as feet, arms, eyes,
head, heart, and tongue. Although bearing the same names, these organs
rarely have a similar shape to those of the vertebrates, being
necessarily adapted to the different mode of living. The foot of a
cockle, shaped like an animal's tongue, enables it to move slowly from
place to place, as well as to burrow in a sandy beach with the comical
jerks so well known to observers. The tongues are beautifully designed
for their work. The long, narrow tongue of the vegetarian mollusc works
like a scythe, and mows down the delicate marine grasses on which the
animal feeds. The powerful tongues of those that prefer an animal diet
are able to bore through the strongest shells; and woe betide the
unfortunate shellfish which, having shown signs of weakness, or disease,
is surrounded by its active, carnivorous brethren. The tongue, sometimes
longer even than the shell itself, is covered with rows of very hard
spikes, or teeth, arranged similarly to the burrs on a file. As these
teeth break, or are worn out, they are replaced by others that push
themselves forward when wanted. Under a microscope of moderate power,
the radula, or tongue, of a shellfish, especially a limpet, is a most
interesting sight, and many molluscs can be identified merely by
examining the tongue under a microscope. The shape of the teeth, the
number, and the arrangement of them will settle the quest