Simple as it is, however, as a mechanical structure, the life and growth of the sponge is as mysterious as that of the most highly organized animal or even the soul of man. We can study out the structure of a plant or animal; we can analyze it and tell what are the elements of which it is composed; we can describe the mechanical operations that are carried on and the chemical combinations that take place, but no man has ever yet solved the mystery of life, even in the lowest form--whether animal or vegetable. The sponge, whether considered as a single or compound animal, has the power to reproduce itself, and here the mystery of life is as much hidden as it is in God's highest creation. It has been stated that every sponge contains a large number of separate cells which carry on the operation of circulation and respiration, and may be likened to the heart and lungs of an animal of a higher creation. Zoölogists claim that each one of these cells represents a separate animal, living in a common structure. However this may be, it is an interesting fact that the sponge has the power of secreting ova that grow in large numbers in little sacks until they have reached a certain stage of progress, when they are expelled from the mother sponge and turned adrift in the great ocean to struggle for their own existence. These eggs do not differ much in their structure and composition from an ordinary hen's egg, except that there is no shell, only a skin provided with little fibers called cilia, that project from it, and by the movement of these the embryo sponge is able to propel itself through the water. It thus lives until it has reached a certain stage of development, when it seeks out a pebble or rock, to which it attaches itself at one end--preparation for which has been made by its peculiar structure during its life when it was free to float around through the water. It is now a prisoner and chained to the rock it has selected for the foundation of its home. Having no longer any use for the little cilia, which enabled it to swim through the water, it now loses them. Here is a beautiful illustration of how nature provides for the necessities of the smallest things, and how when the necessity that demanded a certain condition passes by the condition passes with it. The embryo begins to show a fibrous development, which is the beginning of the framework of a new sponge. Evolution goes on, every step of which is as mysterious as a miracle, until the growing thing is a full-grown sponge, equipped with the means for respiration, circulation, feeding, digestion, and reproduction.