Nervous system and organs of sense.—No nervous fibrils have as yet been discovered in any polygastric animalcule, and, in accordance with this acrite condition, no special instru ments of sensation could, according to all phy siological analogy, be expected to exist; never theless, in many genera the existence of one or two minute spots of a brilliant red colour is conspicuous, which are invariably found to occupy the same position in a given species possessed of them. These red spots are gene rally pronounced to be eyes, although for what reason, except that they correspond in colour with the acknowledged eyes of some of the lowest forms of the Articulata, it is difficult to conjecture. In two species indeed, ( Euglena longicauda and .Amblyophys,) Ehrenberg says he saw a " clear sharply defined ganglion," (einen liellen, scharf umgrenzten-Markknoten,) under the red eye-spot, without, however, offering the slightest proof that the " clear sharply defined body " in question was com posed of nervous matter. Should, however, Ehrenberg's surmises, (for these assertions are nothing more,) be correct, we should indeed encounter in the Infusoria an apparatus of vision of the simplest possible description, con sisting merely of a brain and a coat of coloured pigment, thus dispensing both with the refract ing media that usually constitute an eye, and the nervous communication generally found between it and the brain. Be this as it may, the Polygastria are evidently possessed of considerable perceptive power, (those without red spots quite as much so as those provided with them ;) however rapid their movements, they can steer their course with accuracy, and avoid impinging against each other ; they can likewise perceive the slightest contact, and some species, such as the Vorticellinw for ex ample, exhibit a most exquisite sensibility of touch.
Secretions. — Several species of Polygastria secrete a peculiar fluid of a beautiful violet colour, which is poured into the intestinal canal, where it colours the excrements with which it is expelled from the body. In Nas sula ornata, 16,) the apparatus for secreting this fluid is situated at the anterior part of the body, where it is recognisable as an irregular square spot of a violet colour, situated upon the dorsal surface of the body immedi ately opposite to the dental cylinder. This spot is composed of a great number of little violet globules of unequal size, or, to speak more correctly, of an aggregation of colourless vesicles filled with a violet-coloured fluid. From this spot a canal may be traced running along the back, resembling a string of pearls, in which the violet secretion is conveyed to wards the posterior part of the body. It is only in the posterior third of the body that there seems to exist a direct union between this canal and the alimentary apparatus, for at this point the violet colour of the secretion becomes altered and mixed with foreign matter. In all these Infusoria the violet secretion is expelled through the anal orifice situated at the hinder part of the body, either by itself or in con junction with the excrements. The aggrega
tion of vesicles situated in the back of the neck seems to be the secreting organ of this remarkable fluid, seeing that no vessels could be detected in communication with it, and the surrounding parts were quite transparent and colourless. Ehrenberg believes that the violet liquid, which is of a slightly viscid or almost oily character, possesses some dis solving power, for he has noticed in the alimentary canal of animalcules which con tained a large proportion of it, that frag ments of oscillatorm and other substances taken as food were always discoloured, divided, or decomposed apparently by its action.
Reproduction.— Not the least remarkable feature in the history of the Polygastria is their extraordinary fecundity, which Indeed far ex ceeds that of any other class of animals. The infusorial animalcules, constituting as they do the basis of the great pyramid of the animal creation, the living pasturage diffused through the waters of our globe, on which innumerable creatures have to feed, must be multiplied in proportion to the vast demand for food of this description; and, accordingly, their multiplica tion is effected in various ways, all of which are so prolific that it becomes no longer a matter of astonishment that they swarm to such an extent in every drop of stagnant water, or that their exuvize are found in many localities accumulated in such abundance, that strata of soil and even vast rocks seem to be entirely composed of their reinains.
l'issiparous generation.—This mode of re production consists in the spontaneous fissure of the original animalcule into two or more di visions, each of which soon becomes complete in all its parts and again divides in a similar manner. The 'different steps of this process, which may easily be witnessed, are in ordinary cases as follows. The body of the parent is seen, on its arrival at maturity, to become in tersected by a transparent line, which divides it into two equal halves. In a short time this transparent line becomes indented at each ex tremity, and, as the indentations become more pronounced, the original creature becomes evi dently converted into two, which are united together by a kind Of isthmus, (figs.17 & 18 ) and at length, the isthmus becoming continu ally more and more attenuated, the slightest effort, or the mere action of the vibratile cilia completes the operation, and the two young animalcules, thiis formed, part company and commence an independent existence.
The direction of the line of separation varies in different species, and even in individuals of the same species (fig. 7. 4, 5, 6, 7, 8) ; some times it is transverse, sometimes oblique, and in other cases it traverses the long axis of thebody, where the form of the animalcule is elongated or oval. This method of reproduction is ex ceedingly prolific ; for, as each successive gene ration arrives at maturity in the course of a few hours, and undergoes the same process of divi sion, it will be found on computation that the progeny derived from a single animalcule nzay, in the course of a single month, amount to many hundred millions in number.