Wonderful as is the organisation of the last family, it would probably not be more so than that of the Vibrionidw, was it in our power to display their internal economy in an equally satisfactory manner; but such is the extreme minuteness of all the members of the family, that even to Ehrenberg this seemed a hopeless wish. The VibrionidT present themselves under the rnicroscope as thread-like bodies of in describable tenuity, worming their way in countless thousands through the drop of water in which they live, and presenting themselves in different shapes, which have been classified as belonging to five distinct genera, named as follows :—The first, Bacterium, contains those forms which exhibit the appearance of stiff jointed filaments. In tbe second, Vibrio, the creatures resemble minute chains, which seem to be as soft and flexible as the body of a serpent, although so exceedingly minute that some species have been calculated to be not more than the 300th of a line long, and the 3000th of a line in thickness.
The animalcules in sorne genera assume the appearance of tortuous chains or flexible spiral threads. In Spirillum the body seems rolled into a stiff spiral cylinder, and in Spirodiscus it is arranged in a kind of disc.
On examining these little beings while alive, little doubt can be entertained that they belong to the animal series of creation : the manner in which they obviously direct their course at will, and the facility with which all their movements are performed, have caused them to be recog nised as animals by all observers. It is, how ever, to Ehrenberg that we are indebted for the discovery of their real nature. From his ob servations we learn that these living filaments, minute as they are, are not single animals, but chains composed of numerous associated individuals produced from each other by spon taneous fissure. There even seems to be reason to suspect that their internal structure is in some degree allied to that of the Monadines ; at least in one species, Bacterium triloculure, Ehrenberg perceived a proboscidiform mouth similar to that possessed by tbe Monadines of Volvox.
The peculiar forms assumed by the different genera of Vibrionidm seem to depend upon the character of the fissiparous division by which the whole chain is produced, the compound body remaining straight or becoming thrown into spiral folds as the division is equably or unequably carried on.
The snake-like movements of the trueVibrios during their progress in the vvater, Ehrenberg conceived to be produced by a power of con tracting forcibly, that resides in the individual segments of the compound body, which enables them to change their situation relative to each other.
In the next family, Closterium, (6, 7, fig. 5,) the locomotive organs present themselves under a very different aspect, as, indeed, do the animalcules themselves. The animalcules are incased in a thin, transparent, shuttle-shaped shell, or mantle, (urreolus,) which is in many species evidently open at both ends. Enclosed in this shell is the exceedingly soft and trans parent mucus-like body of the animal, which is frequently entirely full of green-coloured granules and little vesicles. The shell or
mantle, when exposed to heat, is reduced to ashes and entirely volatilized, crisping up during the process like horn.
The locomotive apparatus is exceedingly sin gular in its conformation ; it consists of nu merous very short, delicate, transparent organs, having the form of conical papillm : these are situated in the neighbourhood of the two open ings in the mantle, lying in the inner space, and can be protruded extemally to a short distance. It becomes evident, on mixing a few coloured particles with the fluid in vvhich the animal is contained, that these are instruments of loco motion.
The family Astusia (1, fig. 6) contains nu merous genem remarkable for the contmctile power of their bodies, which causes them con tinually to change their shape, and consequently they become very puzzling objects to the inexpe rienced microscopist. Many of them are exceed ingly beautiful on account of theirrich colours ; and so enormously do they abound under certain circumstances, that the water in which they are found is changed to red, green, or yellow, in accordance with the tint of the species which multiplies therein. In many species of this family, contractile proboscides have been found to exist, which most probably form the loco motive apparatus common to the group. Ani mals very similar to the Astasians, but lori cated, constitute the family Dinobryina, (6, 6,) the envelope forming an urceolus, in which the highly contractile body of the ani malcule is lodged, having much the appearance of a microscopic Sertuluria.
In the next family, Amoeba, locomotion is accomplished in a most extraordinary manner, these animals apparently possessing the power of making foot-like processes for themselves, or dispensing with them altogether, just as cir cumstances render it convenient. The Amoeba, or Proteus, as it was formerly named on ac count of the facility with which it changes its form, seems to have its body composed of a greyish mucus-like jelly, the shape of which is perpetually chang,ing, sometimes shrinking into a rounded mass, then extending itself in all directions as though it was entirely fluid, or shooting out processes of different kinds from any part of the periphery of its body : its movements indeed seem to be rather fluent than progressive, so easily dees it mould itself to any required form. It is, nevertheless, very voracious, and its shape is frequently found to be modified by the contour and dimensions of other animalcules which it may have swal lowed. ( 7, 8, 9, 10, 11, 12, 13,fig. 6.) The genera Difflugia, Arcella, and Cyphidium (1, 2, 3, fig. 7) seena to be merely Amoebw endowed with a power of constructing for them selves a carapax or shelly covering of various forms, from the orifices of which the fluent body of the animalcules can be made to protrude, and thus become convertible into instruments of locomotion.