Protozoa.—The Protozoa have played an important part in rock building. The chalk beds of Kansas and other chalk formations are made up to a large extent of the tiny skeletons of Protozoa, called Foraminifera. Some limestone rocks are also composed in large part of such skeletons. The skeletons of some species are used to make a polishing powder.
Sponges.—The sponges of commerce have the skeleton composed of tough fibers of material somewhat like that of cow's horn. This fiber is elastic and has the power of absorbing water. In a living state, the horny fiber sponge is a dark-colored fleshy mass, usually found attached to rocks. The warm waters of the Mediterranean Sea and the West Indies furnish most of our sponges. The sponges are pulled up from their resting place on the bottom, by means of long-handled rakes operated by men in boats or are secured by divers. They are then spread out on the shore in the sun, and the living tissues allowed to decay; then after treatment consisting of beating, bleaching, and trimming, the bath sponge is ready for the market. Some forms of coral are of commercial value. The red coral of the Mediterranean Sea is the best example.
In some countries little metal images of Buddha are placed within the shells of living pearl oysters or clams. Over these the mantle of the animal secretes a layer of mother of pearl as is shown in the picture.
Pearls and Mother of Pearl.—Pearls are prized the world over. It is a well-known fact that even in this country pearls of some value are sometimes found within the shells of the fresh-water mussel and the oyster. Most of the finest, however, come from the waters around Ceylon. If a pearl is cut open and examined carefully, it is found to be a deposit of the mother-of-pearl layer of the shell around some central structure. It has been believed that any foreign substance, as a grain of sand, might irritate the mantle at a given point, thus stimulating it to secrete around the substance. It now seems likely that most perfect pearls are due to the growth within the mantle of the clam or oyster of certain parasites, stages in the development of a flukeworm. The irritation thus set up in the tissue causes mother of pearl to be deposited around the source of irritation, with the subsequent formation of a pearl.
The pearl-button industry in this country is largely dependent upon the fresh-water mussel, the shells of which are used. This mussel is being so rapidly depleted that the national government is working out a means of artificial propagation of these animals.
Honey and Wax.—Honeybees[29] are kept in hives. A colony consists of a queen, a female who lays the eggs for the colony, the drones, whose duty it is to fertilize the eggs, and the workers.
Cells of honeycomb, queen cell on right at bottom.
The cells of the comb are built by the workers out of wax secreted from the under surface of their bodies. The wax is cut off in thin plates by means of the wax shears between the two last joints of the hind legs. These cells are used to place the eggs of the queen in, one egg to each cell, and the young are hatched after three days, to begin life as footless white grubs.