But by the forms of living creatures is not meant merely their external form. Some general notion then should here at starting be obtained of their internal form—that is, of their essential structure.
The minutest and probably the simplest forms of living creatures (whether plant or animal) are such as are presented by Bacteria,[84] the yeast-plant and Protoccus. Bacteria are those minute creatures the mode of origin of which in sealed infusions has been so much of late disputed, but the activity of which in promoting the decomposition of dead substances is undisputed. A bacterium is a particle of protoplasmic matter, either spheroidal or oblong, or like a short rod, or shaped like a corkscrew, and bacteria may also be in the form of a short chain of spheroids, or of oblong particles, or of rods united in a zigzag manner.
Their breadth may vary from the 1/30000 to 1/10000 of an inch. They may also assume quite another appearance, by surrounding themselves with a gelatinous envelope, which condition is called their zooglæa state of existence.
They may be readily obtained by making some hay tea, and keeping it for a day or two, when they will be found to abound in the scum which forms on the surface, and to be in active motion. In the corkscrew form, Spirillum volitans, each end of the body is produced into a minute hair-like process or cilium, and it is by the lashings of these cilia that the minute organism moves about.
Other as simple but larger organisms may consist of a minute mass of semi-fluid protoplasm, containing granules, as we find to be the case in the plant Vaucheria,[85] and many other Algæ, and in the animal Amœba primitiva.[86]
An organism of this simplest kind or a fragment of a higher organism which presents this simplest condition is called a cell.[87] Very generally such cell has within it a more or less distinctly marked generally denser and spheroidal body called a nucleus, within which, again, other minute spots may appear called nucleoli.
Even in this simplest of all possible conditions of life a slight difference appears between its most external film and its inner substance—just as a cup of broth left to stand will form for itself a filmy outermost layer. This incipient difference between what is inner and what is outer is one which is constantly maintained in all higher organisms, as we shall soon see abundantly. But the distinction into outer and inner is, as has been said, shown in a much more marked way in the constituent units, or cells, which build up the bodies of plants generally; for these consist of an inner part of protoplasm, enclosed in a distinct external cellulose envelope or cell-wall. As has also been shown, many of the lowest animals take on occasionally the encysted condition when they also consist of a particle of bioplasm enclosed in a distinct cell-wall or cyst, though one not made of cellulose.
The protoplasmic contents of the cell may attract watery fluid thus forming clearer spaces or vacuoles within it, and these may become so extended that the protoplasm may be reduced to a thin layer lining the cell wall, thread-like processes or remnants of protoplasm often passing across the cell from one part of the protoplasmic lining to another. A cell, almost always a nucleated cell, is the original form of every living creature without exception; and a great number of small, and some considerably sized living beings, never get beyond this unicellular condition, however much their cell may become enlarged or complicated in shape. Such creatures form the lowest of all animals and plants; but the overwhelming majority of living creatures are formed of aggregations of cells which cohere and fuse together in various ways. As an example of a unicellular and typically cellular living creature we may take the yeast plant (Saccharomyces cerevisiæ), which consists of a particle of bioplasm enclosed in a cell-wall of cellulose, the whole being globular or oval in shape, and generally about 1/3000 of an inch in diameter. Within its bioplasm a clear space or vacuole may often be distinguished. Often these organisms appear with a more complicated outline, due to the growth of new saccharomycetes from its outer wall, and the budding forth of others again from the side of such protruding processes, all of which ultimately become detached as independent saccharomycetes, though they often continue adherent for a long time, forming strings or other temporary aggregations of such organisms.
In Protococcus we meet with one of the lowest order. Its colour is green, which, as in all other higher plants also, is due to the presence in its protoplasm of a colouring matter called chlorophyll, either diffused or aggregated in certain denser granules of protoplasmic substance. Protococcus may be smaller or much larger than the yeast plant, it is spheroidal, and its protoplasm is enclosed in a tough case of cellulose, which, however, it may not nearly fill, while the long cilia may protrude through it and propel the whole organism by their reiterated lashings.
It has been already said that a vegetable may temporarily exist as a particle of bioplasm without any cell-wall, and such is the case with Protococcus, the cellular envelope of which occasionally disappears. More remarkable still is the form already referred to under the name Myxomycetes,[88] which, for part of its existence, is the form of an indefinitely-shaped, naked protoplasmic mass.[89]