In addition to the cilia some forms of ani malcules ( Oxytrichina) possess setce, which are likewise stiff moveable hairs, but which are without any power of vibration ; these organs are used in standing and climbing. Sometimes they are without any thickened basis, as in Actinophrys ; generally they are pointed, but occasionally have a knob at the end.
A fourth set of locomotive organs are the styli. These are thick straightsetm which in some fol ms of animalcules are attached like the tail fea thers of a bird to the hinder part of the body ot the animalcule : such styli do not vibrate like cilia, neither are they implanted in a bulb-like basis, nor bend like hooks, but serve merely as instruments of support, or are useful in climbing the stems of aquatic plants.
Lastly, many races are furnished with uncini or hooklets ; these are merely bent, hook-like seta>, which, being thick and strong, and situ ated upon the ventral surface of the animal cule, seem to take the place of feet : they do not vibrate, but are implanted into a bulb-like root, which permits them to be moved in all directions; and although they are not articu lated, they resemble very much the limbs of articulated animals.
So various, however, are the forms of the different families of Polygastric animalcules, that the above general view of their locomotive organs gives but a very imperfect idea of this part of their economy; and it will, therefore, be ne cessary, before we proceed further, to describe more at length some of the most interesting ge nera belonging to the class, for so strange and re markable is the organisation of some of them that no generalisation would answer our present purpose. Some are single and isolated indivi duals, moving freely wherever they list ; others are strangely compounded of aggregations of numerous animalcules associated into one com mon body, all of which must cooperate in rowing about the microcosm which they col lectively form; some are affixed to highly irri table stems, whereby they are attached to various foreign bodies; some are naked, others covered with shells: in short, nothing but a rapid glance at the whole group will enable us satis factorily to discuss the many curious circum stances discovered in connection with their history.
The family MONADINIDN embraces nume rous animalcules, which, however different in external appearance, are evidently related to each other in all essential parts of theii struc ture.
The Monads, properly so called, are so small that the utmost penetration of the miscroscope is insufficient to display their outward form with any degree of distinctness, much less to reveal their internal structure, some of them being not larger than from the 1,000th to the 3,000th of a line, or the 36,000th part of an inch in diameter. Under the highest powers of the microscope they have the appearance of almost invisible globular active specks, swim ming about with the greatest facility, and never impinging against each other during the rapid dance that they continually execute. Their numbers are absolutely beyond human appre ciation, as may be readily understood from the following computation of the multitudes some times met with.
The Illonas crepusculum, found in infusions of putrid flesh, crowds the drop of water in which it is found to such an extent that there seems to be no interspace whatever between the individual animalcules. Supposing these animalcules to be, as is generally the case, 4„th of a line in diameter, their number will then amount, in a drop of water of tile size of a single cubic line, to eight thousand millions, and a cubic inch of such water containing 1728 cubic lines, will be peopled with thirteen billions eight hundred and twenty-four millions of these living and active beings !!! It has been possible to detect, even in these smallest of nature's works, an apparatus that seems to perform the functions of an instrument of progression. This consists in one or some times two filaments of extreme tenuity, which resemble somewhat the tail of a tadpole ; here, however, the organ performs the functions of a proboscis, being appended to that part of the body which advances first in swimming. The shape of the Monads is not always globose, but sometimes egg-shaped, pear-shaped, elon gated, or fusiforin. In Monas tingens we have an example of the last form, and also of the manner in which they are sometimes found associated by their tails into beautiful groups, their double proboscides being all protruded externally.