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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