Consequently we distinguish among the simplest living things those which are provided with leaf-green, and feed, as do the larger green plants, on dissolved “mineral” solids and gases. There are many thousands of kinds of them—single simple cells. Some are known to microscopists as Diatoms and Desmids—often of curious spindle or crescent-shape, others star-like. The diatoms form on their surface a delicate, wonderfully-sculptured coat of glass-like silica (quartz), which resists destruction and persists long after the protoplasm is dead and washed away. They are favourite objects for examination with the microscope on account of their great beauty and variety.
Those simplest living things which have not got leaf-green to enable them to feed on mineral food must—unless they are parasites (as many important kinds are)—get their food, as do bigger animals, by feeding on the solid substance of other living things. All living things are, in fact, ultimately dependent on the green plants—whether microscopic or of larger kinds—not only for food, but for oxygen gas. If you could take away green plants altogether from the world, the animals would eat one another and use up the oxygen gas of the atmosphere, and at the last there would be a few only of the strongest left, like the last survivor of the shipwrecked crew of the Nancy Bell, and even they would be suffocating for want of oxygen. The single cells, which are independent animalcules, and feed like animals on whole creatures smaller than themselves, or on bits of the fresh substance of other animals or of plants, are of extraordinary diversity of form and activity. Unlike the unicellular plants, whose food is dissolved in the water in which they live, the single-cell animals of necessity take their food in “lumps” into their inside and digest it, and so their cell-protoplasm has either a soft surface which can take up a food-morsel at any point or it has a firm surface with a definite mouth, or aperture, in it (see [Fig. 41]) where the mouth is marked by an arrow. Many of them, especially those with soft glutinous protoplasm, which extends from the main-mass in long threads or branching processes searching for food-morsels, form marvellous, perforated shells by chemical deposit, either of silica or limestone (Radiolaria and Foraminifers). The kinds with a firm or tough surface to the cell-protoplasm and a permanent mouth and gullet leading into the cell-substance have very usually a single large lashing-whip (Flagellata), which drives them through the water in search of prey, or they are clothed with hundreds of such lashing threads of smaller size—the “cilia” described above ([p. 195])—arranged in rows or circles, whence these animalcules are called “Ciliata.” The ciliates or one-celled animals are enabled by their cilia to move with all the grace, variety, facility, and apparent intelligence of the highest animals, and also to create powerful vortex-currents by which food particles are driven into the cell-mouth.
It is a most remarkable and thought-stirring fact that here we have “animalcules” which are no more than isolated units of the kind and structure which go by hundreds of thousands to build up a larger animal—just as bricks are units of the kind which to the number of many thousands build up a house. And yet each of these free-living units has a complete organisation—mouth, pharynx, renal organ, locomotive organs, and so on—similar in activity and general shape to the system of large, capacious organs built up by the agglomeration of millions of cell-units to form the body of a higher animal. It is as though a single brick were provided with door, windows, staircase, fireplace, chimneys, and wine-cellar! It is clear that there is only a resemblance and not an identity of origin between the organs of the multicellular animal and those of the single-celled animalcule. The history of the growth of an animal from the single egg-cell, and also the series of existing many-celled animals, leading from simple forms to the most complex, proves this. And in view of that fact the wonderful elaboration of these diminutive creatures—many of them so small as to be absolutely invisible to the naked eye—is all the more curious and impressive. We have, in fact, parallel organisation and elaboration of structures with special uses, in two absolutely separated grades or strata of living things—the one grade marked off by the limitation that only a single cell, a single nucleated corpuscle of protoplasm, is to be the basis and material of elaboration—the other and higher grade permitting the use of millions of single cells, of endless variety and plasticity, capable of hanging together and being grouped in layers and tissues, in such enormous masses that an elephant or a whale is the result. And we see that the same needs are met, not actually in the same way, but in the same kind of way, in the two cases—the food-orifice, the cilia, and the “pulsating vacuole” of the unicellular animalcule do the same services as those done by the structurally different mouth, legs, and kidneys of the elephant.
[XXII]
TADPOLES AND FROGS
The season of tadpoles is not a season recognised by housekeepers and gourmets (except in France, where frogs are eaten in April), but one dear to schoolboys and all lovers of Nature. The ponds on heaths and in the corners of meadows now show great masses of soft jelly-like balls of the size of a marble, huddled together and marked each by a little black spot at its centre, as big as a rape-seed. This is the “spawn” of our common frog. The spawn of the common toad is very similar, but the black spots are set in long strings of jelly, not in separate balls. The little black body is precisely the same thing as the yellow part of a hen’s egg, and the jelly around it corresponds to the “white” of the bird’s egg; but there is nothing to represent the shell. The “yelk” of the bird’s egg is, it is true, much larger, but corresponds to the black sphere of the frog’s egg—the actual germ—and is like the latter a single protoplasmic cell, distended with nourishing granular matter. It is the excess of this matter which makes the yellow ball of the bird’s egg so much bigger than the black or rather deep-brown germ of the frog. The little black spheres elongate from day to day in the warm spring weather, and at last the minute tadpoles (see [Fig. 43] and its explanation) break loose from the jelly, hanging on to its surface by aid of a tiny sucker, and feeding on the minute green vegetable growths which have appeared all over the jelly-like mass. Their rate of growth depends very much on the temperature, and is much more rapid in Italy and the South of France than in England. At first they are so small that it is difficult to distinguish, except with a pocket-lens, the little black plume-like gills on each side of the head, and it is only as they grow bigger and lose these little plumes that the young things assume the characteristic shape of a rounded head—really head and body—with a long flattened tail which strikes vigorously to the right and left, and enables the tadpole to swim like a fish.
Fig. 43.—Stages in the growth from the egg of the common frog—drawn of the natural size. 1. Egg in its jelly-like envelope. 2. Very young tadpoles adhering to weed by their suckers (placed just below the mouth). 3. Very young tadpole, showing two pair of external gills: a third pair is present, but so small as to be invisible without magnification. 4, 5, 6. Stages in the later growth of the tadpole: the external gills have disappeared, but the legs have not yet made their appearance. 7. Tadpole of full size, with fore and hind legs. 8. The tadpole has now become a small frog, and has left the water. The tail has shrunk, but has not entirely disappeared: it remains throughout life hidden by the skin and the large thighs of the growing frog. This figure has been kindly supplied by Messrs. Macmillan & Co., from Dr. Gadow’s volume on the “Amphibia and Reptiles,” in the Cambridge Natural History.