As long as the animalcule continues free from annoyance, the trichocysts undergo no change; but when subjected to external irritation, as occurs during the drying away of the surrounding water, or the application of acetic acid or other chemical irritant, or the too forcible action of the compressor, they become suddenly transformed into long filaments, which are projected from all parts of the surface of the animalcule; these filaments have been mistaken for cilia by Cohn and Stein. The rapidity with which their evolution is effected, joined with the great minuteness and transparency of the object, renders it extremely difficult to follow it.

(71). It is not difficult, by rapidly crushing the animalcule, to force out some of these organs in an unchanged state. If the eye be now fixed on one of the isolated trichocysts, it will most probably be seen, after the lapse of a few seconds, to become all at once changed (with a peculiar jerk, as if by a sudden release from some previous state of tension) into a little spherical body. In this condition it will probably remain for two or three seconds longer, and then a spiral filament will become rapidly evolved from the sphere, apparently by the rupture of a membrane that had previously confined it, the filament unrolling itself so quickly that the eye can scarcely follow it, until it ultimately lies straight and rigid on the field of the microscope, looking like a very fine and long acicular crystal.

(72). This remarkable filament, when completely evolved, consists of two portions - a rigid spiculum-like portion, acutely pointed at one end, and continuous at the opposite end with the second portion, which is in the form of an excessively fine filiform appendage, less than half the length of the spiculum. This second portion is generally seen to be bent at an angle on the first, and is frequently more or less curved at the free end. The form of the evolved trichocysts is best observed in such as have floated away towards the margin of the drop of water and are there left dry by the evaporated fluid. In many of them the filiform appendage is not visible, and they then merely present the appearance of a simple, long, fusiform spiculum.

(73). Few subjects have afforded a more fertile field for discussion than the internal organization of these almost invisible creatures; and even at the present moment many points of their economy are by no means satisfactorily elucidated. The remoteness of their structure from that of the higher animals, and a natural mistrust felt by recent observers in the capabilities of the instruments placed at their disposal, gave rise at first to doubts and hesitation, which no longer exist.

The digestive apparatus of the Infusoria was originally described by Ehrenberg as consisting of a number of internal sacculi, varying from four to two hundred in number in different species. These sacs were stated by that indefatigable microscopist to be readily distinguishable without any preparation, but capable of being rendered more conspicuous by feeding the animalcules with pure carmine or indigo, the coloured particles of which substances they eagerly swallow. In one large division, called Anentera, the sacculi or stomachs were said to arise by separate tubular pedicles from the mouth itself (fig. 16, l); whilst in others (Enterodela) there was supposed to be a complete intestinal canal, terminated by a mouth and anus, to which the sacculi or stomachs, as they were called, are appended: sometimes the intestinal canal is stated by the same authority to form a circle in the body Anopisthia, Ehrenb.), as in the Vorticella (fig. 16, 2); or else the mouth and anus are placed at opposite extremities of the body, through which the intestinal tube passes either in a straight course, or exhibiting several flexuous curves in its passage (Enantiotreta and Allotreta, Ehren.) (fig. 16, 3 and 4.) When neither the mouth nor anus is terminal, such animalcules belong to the group denominated Katotreta by the same author.

(74). However imposing, from their completeness, the views of Ehrenberg concerning the digestive system of the polygastria may be, and sanctioned as they have been by almost general consent, we cannot pass over a subject of so much importance without expressing ourselves as being far from admitting their accuracy, and we must say that our own observations upon the structure of the polygastria have led us to very different conclusions*.

(75). The positions of the mouth and anal aperture we are well assured, by frequent examination, to be such as are indicated by the illustrious Professor of Berlin; but with regard to the tube named by him intestine 1, and the stomachs appended thereto, our most patient and long-continued efforts have failed to detect the arrangement depicted in his drawings. In the first place, as regards the function of the sacculi, which he looks upon as the organs in which digestion is accomplished: in carnivorous animalcules, which devour other species, we might expect, were these the stomachs, that the prey would at once be conveyed into one or other of these cavities; yet, setting aside the difficulty which must manifestly occur in lodging large animalcules in these microscopic sacs, and having recourse to the result of actual experience, we have never in a single instance seen an animalcule, when swallowed, placed in such a position, but have repeatedly traced the prey into what seemed a cavity excavated in the general parenchyma of the body.

(76). In the second place, the sacculi have no appearance of being pedunculated and consequently in a certain degree fixed in definite positions: we have just been for two hours carefully examining some beautiful specimens of Paramecium Aurelia (fig. 16,4), an animalcule which, from its size, is peculiarly adapted to the investigation of these vesicles; and so far from their having any appearance of connexion with a central canal, as represented in the figure copied from Ehrenberg, they are in continual circulation, moving slowly upwards along one side of the body, and in the opposite direction down the other, continually changing their relative positions with each other.