The Rotifera, undoubtedly, deserve to be called Infusoria as much as the Polygastria, as they are found very generally with the latter m various kinds of infusions. There are some circumstances, however, under which the Polygastria are developed, in which no Rotifera have yet been found : thns the Polygastria have been found inhabiting water, containing sulphuretted hydrogen and other gaseous constituents, where no Rotiferous ani malcules have been found at all. As a general statement, it is true that the Rotifera are the last to appear in infusions ; but there are many instances in which Polygastria are developed without the subsequent appearance of Rotifera, and they disappear from infusions sooner than the former. Of the 722 species of Infusoria, described by Ehrenberg, he found that forty one only were commonly present in the various artificial infusions, which he made in various parts of the world. Of these only three species belonged to the class Rotifera, viz. Co lurus uneinatus, Icthydium podura, and Lepa della ovalis. It is the appearance of these anirnalcules in infusions, which among other things have led to the question of equivocal ge neration (GENERATION); but whateverground the low organisation of some of the Poly gastria might afford for a belief in this doc trine, the Rotifera have an organisation too high to allow of doubt on this point. The fact of creatures so highly developed being produced in infusions, would create a doubt with regard to the whole theory of equivocal generation, which only positive observation could set aside.
The Rotifera, although classed with the Po lygastria as " infusory animalcules," must n ot be regarded as performing a common function with them in the economy of creation, for not only are there fewer species of Rotifera, but they also exist in much smaller numbers.
Whilst the Polygastria descend in structure to a point where it may be well questioned, whether they partake most of the animal or vegetable character, the Rotifera have always a decided animal character. The Polygastria are even said to perform functions, such as the absorption of carbonic acid and the evo lution of oxygen, which would seem to throw doubt on their animality altogether ; but no such function can possibly be attributed to the Rotifera. They appear to be distributed as widely on the earth as the Polygastria; and Ehrenberg has recorded their existence in various parts of Europe, Asia, and Africa. They have also been found in America. They inhabit both salt water and fresh, although the species which inhabit the latter are by far the most numerous. Like some of the higher animals, the same species are found inhabiting both salt and fresh water, whilst others are peculiar to brackish water. Although they are capable of pursuing their way in the open water, they are generally found swimming around, or attached to, the leaves and other parts of aquatic plants. In our own country the leaves of Ceratophyllum are found to be a favourite resort of species of Lininias, Masti gocerca, Dinocharis, Monura, and others. The floating roots of the various species of Lemna are also the favourite resort of several species, whilst others are found in abundance amongst the fibrilliform fronds of the fresh-water algw. Some of them even take up their residence in the interior of the cells of plants. Roper first discovered them in the cells of Sphagnum obtusifolium. Subsequently Unger described
a peculiar movement in certain tubercles which he had observed to be developed upon the stalk of Vaucheria clavata. The same phe nomenon was witnessed by Professor Morren, of Liege, who, on investigating the subject more closely, found that the movements of the tubercles was due to the presence in their interior of the Rotifer vulgaris. Others, again, are found in turfy and bog waters ; whilst some, especially the species of Notommata, are found parasitic upon other animals.
The Rotifera are more susceptible to the in fluence of either high or low temperatures than the Polygastria. Ehrenberg observed the latter constantly come to life after the water in which they were contained had been frozen. Species of Diglena, , Colurus , and Lepadella frequently came to life after they had been frozen for a short time. Other species experimented on, as Hydatina senta, Brachionus urceolaris, and species of Salpina, all died. Although they are easily destroyed by being frozen, some of them will bear a great variety of temperature. Thus the Phi lodina roseola, which we have found in the streams of Yorkshire, has been discovered by Professor Agassiz amongst the red snow of the Alps, where it must have been exposed to a much lower temperature than in the former habitat. Polygastria bear also a higher de gree of heat than Rotifera. Brachionus 217Te0 laris and Hydatina senta were found alive after having been exposed for thirty seconds to a temperature of 104° Fah. Higher tempera tures speedily destroyed them.
One of the most remarkable points in the economy of the Rotifera is the power they possess of recovering their vitality after having been apparently perfectly desiccated. This fact was first made known by Leeuwen hoek, who, at the same time that he disco vered the existence of the common Rotifer, had an opportunity of observing this remark able property. In one of his original papers, contributed to the Royal Society of London, he says :— " In October, 1702, I caused the filth or dirt of the gutters, when there was no water there, and the dirt was quite dry, to be ga thered together, and took about a teacupful of the same and put it into a paper upon my desk, since which time I have often taken a little thereof, and poured upon it boiled water, after it had stood till it was cold, to the end that I might obviate any objection that should be made, as if there were living creatures in that water. These animalcula, when the water runs off them or dries away, contract their bodies into a globular or oval figure. After the above-mentioned dry substance had lain near twenty-one months in the paper, I put into a glass tube, of an inch diameter, the remainder of what I had by me, and poured upon it boiled rain water after it was almost cold, and then immediately viewed the small est parts of it, particularly that which sub sided leisurely to the bottom, and observed a great many round particles, most of which were reddish, and they were certainly ani malcula ; and some hours after I discovered a few that had opened or unfolded their bodies, swimming through the water ; and a great many others that had not unfolded them selves, were sunk to the bottom, some of which had holes in their bodies ; from whence I concluded that the little creature ailed the mite had been in the paper, and preyed upon the aforesaid animaleula.