Before entering upon the great subject of water and ice—two of the most tremendous factors in world-building—let us consider a small matter, so far as its permanent effects are concerned, yet one which enters largely into the comfort and health of mankind, and which, though an animal, may be discussed where it belongs—under "Water."
There are few things more familiar about the ordinary household than a piece of sponge, and yet, perhaps, there are but few things about which there is so little known. The sponge had been in use many, many years before it was given a place in either the animal or vegetable kingdom. The casual observer, because he saw it attached to a rock, jumped to the conclusion that it was of vegetable origin. But after being kicked back and forth, so to speak, from one kingdom to the other, even by what are called well-educated people, it has finally been received into the family of animals; a dignity in which the sponge itself seems to take but little interest.
The sponge is found in the bottom of the sea; at no very great depth, however. It is usually attached to a rock or some other substance and it is due to this fact chiefly that it has been classed as a vegetable. At least one scientist has attempted to give it a place between the two kingdoms, but this only adds confusion without giving any satisfactory explanation of its origin. It seems to belong to a very low order of animal life. It breathes water instead of air, but probably, like many other water animals, it absorbs the oxygen from the air which is more or less contained in the water. There is a process of oxidation going on within the sponge in a manner somewhat as we find it in ordinary animal life, and like the animal it expels carbon dioxide. All this, however, is carried on apparently without any lungs or any digestive organs, or in fact any of the organs that are common to the animals of the higher order. The sponge, however, as we see it in our bathrooms, is only the framework, bony structure, or skeleton of the animal.
The sponge is exceedingly porous and readily absorbs water or any fluid by the well-known process of capillary attraction. The sponge fiber is very tough and is not like anything known to exist in the vegetable kingdom. The substance analyzes almost the same as ordinary silk, which all know is an animal product. If we burn a piece of sponge it exhibits very much the same phenomena as the burning of hair or wool, and the smell is very much the same.
The structure of a piece of sponge when examined under a microscope is a wonderfully complicated fabric. Under the microscope it shows a network of interlacing filaments running in every direction in a system of curved lines intersecting and interlacing with each other in a manner to leave capillary openings.
It is a wonderful structure, and one that a mechanical engineer could get many valuable lessons from. It will stand a strain in one direction as well as another. There are no special laminations or lines of cleavage; it is very resilient or elastic, and readily yields to pressure, but as readily comes back to its normal position when the pressure is relieved. If we examine the body of a sponge we shall notice that there are occasional large openings into it, but everywhere surrounded by smaller ones. If we should capture a live sponge and place it in an aquarium with sea water, where we could study it, we should find a circulation constantly going on, and that water was constantly sucked in at the smaller openings all over the outside of the sponge and as continuously ejected from the large openings. This process constitutes what corresponds in the higher order of animals to both respiration and blood circulation, combined. The sponge feeds upon substances that are gathered up from the sea water, and breathes the air contained in the same, so that it breathes, eats, and drinks through the same set of organs.
When we first capture a live sponge from the sea it has a slimy, dirty appearance, and is very heavy. The sponge is found to be filled with a glutinous substance that is the fleshy part of the animal. It is very soft and jelly-like, and after the sponge is dead it is readily squeezed out, by a process which is called "taking the milk out," which leaves simply the skeleton, the only useful part as an article of commerce. This fleshy substance, in life, has somewhat the appearance and composition of the white of an egg.
The mechanical process by which the sponge takes its nourishment is exceedingly interesting. There are small globe-shaped cells with openings through them that are lined with little hairlike projections that move in such a manner as to suck the water in at one side of the cell and push it out at the other. These little fibers are technically called "cilia." We might describe them as little suction pumps that are located at many points in the sponge, all acting conjointly to produce a circulation through the finer openings or capillary vessels and finally discharging into the larger chambers which carry off the residue. If we should analyze the water as it is sucked into the sponge and that which issues from it through the larger openings, we should find a difference between the two. The expelled water would contain more or less carbon dioxide.
There are many different varieties of sponge, and, while they all possess certain characteristics in common, they are still very different in many respects. Some of them are large and coarse, while others are exceedingly soft and velvety. What is called a single sponge is a colony of animals rather than a single animal; at least they are so regarded by zoölogists. This can hardly be true if we regard the sponge itself as a part of the animal. If the sponge is simply regarded as the house in which the animal lives then it becomes a great tenement with numerous occupants. But it is a tenement upon which the life of the sponge depends, and is a part of it.
The sponge could not breathe without the fibrous structure in the cells containing the machinery for producing the circulation. It will be seen that the sponge, while it is an animal, is of the very simplest variety, so far as its organs are concerned. True, its framework is very complicated, but the organs for sustaining the life of the animal are the simplest possible. The little self-acting pumps pull the water into the sponge through the smaller openings, where it appropriates the food substance from the water and where a chemical action takes place which builds up the fleshy substance of the animal, and then expels the residue which is not needed to support its life.