# Midland Naturalist/Volume 01/Freshwater Life.—III, Infusoria

For part 1, see Freshwater Life.—1, Entomostraca; for part 2, Freshwater Life.—2. Rotifera
Freshwater Life.—III, Infusoria.
By Edwin Smith, M.A.
If a little hay is steeped in water fora few days, and the infusion is then examined, it will be found to teem with microscopic life. Similarly the natural infusions offered by accumulations of water containing decaying animal and vegetable matter, or ponds where the simpler forms of vegetation flourish, are tenanted by countless millions of minute animals of various kinds, which, from their mode of occurrence, were named by the earliest observers Infusoria. The term at first included many organisms which, further investigation showed, could not be retained in the same class; plants mistaken for animals, because they moved about; animals of higher organisation, such as the Rotifera; others of a lower type, like the Amœba. This sifting process is even yet far from complete. It is not improbable that many forms now placed in this class, when their life history comes to be written, will have to be separated from the Infusoria properly so called. The number of species may also be reduced, For in the imperfection of our knowledge it is well to remember that forms which to-day are counted as distinct species may hereafter prove to be only different stages of the same animal.
Like other members of the sub-kingdom Protozoa, the creatures we are considering possess a simple body net divided into segments, and one which cannot be cut into two exactly corresponding halves. There is no definite alimentary canal, hut digestion is effected indifferently in any portion of the fluid contents of the body. Pellets of food may he lodged in vacuoles extemporised in various parts of the interior, but they are not enclosed in stomachs separated by any sort of wall from the surrounding mass. Compared with other Protozoa, the Infusoria exhibit a more advanced differentiation of structure. The fluid protoplasm or sarcode, of which the bulk of the body consists, passes externally into a denser portion, the so-called cortical layer; which again is often protected by a still firmer covering termed the cuticle. Food is admitted by a distinct ciliated month opening into a short ciliated gullet, whence it passes, together with a small quantity of water, into the general body-cavity. When a proper month is not present, there is at least an oral region where a mouth may be extemporised. Refuse is excreted at a particular spot situated near to or remote from the mouth; but the discharging orifice is not, as a rule, permanently visible. One, two, or more contractile vesicles, having a fixed position in the cortical layer and connected with channels leading inwards, serve by their slow expansion and quick contraction to keep up a sort of circulation in the fluid interior, and to purify the sarcodic contents. In the same layer are found a nucleus, the female element, and, attached thereto, a nucleolus, the male element of the reproductive process, Reproduction takes place either by self-division lengthwise or crosswise, or by the conjugation of two individuals; the former method being characteristic of the sedentary, the latter of the free-swimming Infusoria, The same species may multiply in both ways.
​With regard to external appendages, the three sub-groups of Infusoria are differently furnished. The Flagellata have one or two long filaments like a whip-lash; the Acinetæ have numerous radiating tubular tentacles, which act as suckers; and the Ciliata have vibratile cilia, employed as organs of prehension and locomotion, In the last group the cilia may be distributed over the whole body and be all of one kind, or widely scattered over the surface and of different kinds, or limited to the wader side of the body, or placed in a circular or short spiral round it, in some species the cilia will move in concert when the animal swims. In others only the smaller cilia at certain points appear to assist locomotion, while the larger resemble non-motile bristles and only occasionally move by the twitching of the superficial coat. Certain bell-shaped forms, e.g., Vorticella, possess a tubular stalk by which they are attached to water-plants during the principal stage of their existence. This stable contains a contractile thread, which, on the slightest shock, shortens into a spiral and jerks back the bell.
Of the ciliated kinds some have the power of secreting a soft gelatinous envelope or an open vase-like case, into which they retract and from which they extend themselves at pleasure, The case is mostly fixed by a very short stem, or by its closed end to some aquatic plant; but occasionally it is carried about by the animal, which has broken away from its support, and swims freely through the water. Many Infusoria, possibly all, undergo, at certain periods, what is called the encysting process; that is to say, they enshroud themselves for a time in a gelatinous covering, which hardens into a thin membrane, and meanwhile they become quiescent. The process appears to be subservient to preservation through the cold season, or to multiplication, or to metamorphosis.
To the above brief description of the class I will now subjoin a few notes on such common examples as have secured to me in the ponds about Nottingham. Stentor may be looked for at all seasons in clear water covered with duck-weed. I have taken specimens in February during frosty weather. The body is trumpet-shaped when extended, but is drawn up at times into various figures from obtusely conical to ovate or globular. It often fixes itself by the narrow extremity, which spreads out a little adhesive foot for the purpose. When free, it swims with a moderately quick rolling motion. The surface of the whole body is covered from end to end with lines of minute cilia; while a broken spiral of longer ones surrounds the head with a wreath like a figure of six. The break in the wreath indicates the position of the mouth, near which may be noticed a contractile vesicle and its connected channels, I have occasionally seen three or four transparent vacuoles at a time bulging out from the side of the animal, but they did not contract. The nucleus, when I have observed it, has had the form of a horse-shoe. As to colour, my specimens have been either pellucid, or tinted dull blue, green, or even black, by their granular contents. The last were evidently specimens of S. niger, and were got from a pond in Bestwood Park; length, ⁠1/26⁠ of an inch. It is interesting to notice diatoms, small animalcules, and so forth, in the foot vacueles. Instances of self division ​or conjugation may often be observed: and I once met with a whole colony, produced by progressive self-division, all thrusting out their heads from the muddle of a limp of jelly, which served as their common envelope.
The bell-shaped Vorticelllæ are familiar to every microscopist, and they are as beautiful as they are common. They are often so abundant as to look like a white fluff clothing the roots and stems of aquatic plants. The bell has no cilia on its surface; but from the open rim protrudes a disk which beats a rotary wreath of these organs. In the depression between rim and disk lie the month and the excretory orifice close together. The former opens into a well-defined gullet, which extends some way into the interior, where also a contractile vesicle and a curved nucleus may readily be discerned. Careful illumination is needed to show the contractile thread inside the tubular stalk. When a Vorticela breaks away from its place of attachment, as not infrequently happens, the bell may swim of with the stalk in tow. In one such instance I observed the bell come to rest by its cilia on a bit of weed; and while in that position, the stalk every now and then contracted spirally as usual, although the movement could be of no possible use to the creature. This led me to think that the movements is ordinarily quite independent of anything of the nature of will. Specimens may be met with in various stages of fission; and occasionally one or two small oval bodies are found adhering to the stalk where it joins the bell, but what part they play in the life-history of Vorticella I have not been able to witness.
Continuous self-division increases the number of individuals by a sort of geometrical progression. In such a way are probably formed those splendid compound clusters which, as in Carchesium polypinum, exhibit the magnificent spectacle of forty or fifty bells connected by their ramifying threads with one common trunk. It is a fine sight to behold a number of bunches all contract their fibres at the same moment to one centre, the top of their common pedicel, and to spread out again in loose array as before; and to see this done again and again, not by one specimen alone, but by a colony of specimens crowding the bit of water-plant under examination. I have taken Carchesium on Ancharis from under the ice in the month of January.
Minute Vorticelline forms are found in parasitic clusters on the carapaces of Cyclops, Daphnia, and other Entomostraca; on the shells of water-snails, on water-beetles, and on various aquatic larvæ. Not that the so-called parasites actually feed upon the substance of their host; they do not claim board, but only lodging. They feed in the surrounding element, as usual, by their cilia. Epistylis digitalis infests in thick masses the abdomen of Cyclops, having the appearance of an elegant but rather cumbersome train. The bell-part is ⁠1/136⁠ of an inch long; the little stalk is branched and non-contractile, A much smaller species, with a simple stalk, is a length of no more than (illegible text) of an inch, I have also met with a sessile form, filled with grains of chlorophyll, and completely colouring the abdomen of the unfortunate Cyclops green. Length of body from (illegible text) to (illegible text) of an inch.
​There is a family of Infusoria closely allied to the Vorticellæ; In their form and habits, but distinguished by the absence of a stalk and by the possession of a roomy sheath resembling a very thin transparent vase, into which the animal can withdraw itself by a general contraction of the whole body. The two kinds with which I um acquainted are Vaginicola and Cothurnia. The sheath of the former is sessile; that of the latter is supported on a short stem. They multiply by self-division. Hence they frequently occur in pairs, each pair occupying a common vase. From the vase the twins protrude their ciliated crests by a simultaneous impulse, and after feeding awhile suddenly close up and shrink to the bottom of their cell. The retreat, however, is only momentary. Immediately they begin to rise again slowly and steadily, until they once more stretch forth to fish for prey, A contractile vesicle is situated not far from the mouth. The sheath of Cothurnia may be yellow. brown, or sty red. I have seen Vaginicola sailing about, case and all, attached to a fragment of weed. Both kinds continue active through the winter.
[To be continued.]
This work was published before January 1, 1930, and is in the public domain worldwide because the author died at least 100 years ago.
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