These have little in common with the order of infusoria of which we have spoken, being both more highly organized and formed on a different plan. Even in respect to size they differ, being generally much larger, some having a length of half a line, and many being within the limit of unassisted vision. By many naturalists they are classed with the articulated animals, under the term of cilio-articulates. Their name, as we have already stated, is derived from a particular and very curious arrangement of the cilia covering two lobes near the anterior extremity, which when in motion have exactly the appearance of two minute wheels rotating very rapidly. But this, though a striking peculiarity of many rotifers, is not common to them all. In some the cilia about the head are arranged in a wavy line. The rotifera may be defined as minute worm-like animals, very transparent, without legs, having the anterior portion of the body furnished with certain retractile lobes, the margins of which are covered with cilia, the alimentary canal distinct and having two orifices, the mouth having a true dental apparatus, the reproduction by ova only. They are aquatic, though a few species can exist in moist earth.

They are found alike in salt and fresh water, but rarely in that which is rendered foul by decaying vegetable and animal matter, and which swarms with the polygastric animalcules. It is only when these have devoured the decaying matter that the rotifer appears to feed upon them. Rotifera have great tenacity of life, and are not destroyed by complete and long-continued desiccation. Individuals have been kept in vacuo with sulphuric acid and chloride of lime, insuring the utmost possible amount of dryness, for a month, and yet revived on being placed in water. The rotifera have always two investing membranes, both transparent, and the inner always flexible; the outer is in many quite firm, constituting a horn-like tube, from which the head and tail of the animal protrude. It never contains either lime or silica, which is probably the reason why no traces of these animal forms are found in any fossilif-erous rocks. Their bodies are retractile, and many creep like worms. They swim by means of their cilia very rapidly. Near the tail is, in most forms, either a dirk-like or claw-like process, by which the animal can attach itself.

The gullet is furnished at its inferior portion with a masticating apparatus consisting of two strong semicircular jaws, each furnished with from one to five teeth, which appear to contain mineral matter. The stomach is either globular or tubular, and scarcely distinguishable from the intestine below. Near the anus the intestine is enlarged into a sort of cloaca with which the genital apparatus communicates. Several small glandiform bodies are observed near the alimentary canal, and some undoubt-edly communicate with its cavity. It is a curious fact that, though the digestive apparatus is in most of these animals much more fully developed than any other, yet in one genus described by Mr. Dalrymple ("Philosophical Transactions," 1819, p. 339), no anal orifice was found, and indeed scarcely any intestinal canal; so that the excrementitious food must have been ejected from the mouth, as in some of the very low polygastric forms. - We now come to locomotion. Several distinct longitudinal bands of a highly contractile tissue pass the entire length of the animal, and certain transverse bands have probably the same power. It is, however, very doubtful whether any true muscular tissue, with the characteristics by which we identify it in the higher animals, exists in these animalcules.

The same remark applies to the nervous system. The function is certainly performed; but whether the cords and masses which Ehrenberg describes as nerves and ganglia really have that character, is at least uncertain. Two red spots near the head are supposed on pretty strong evidence to be eyes, or at least rudimentary forms of the organ of vision. There is no proper circulatory apparatus, but water is very freely admitted into the body, and probably serves to aerate the tissues. It is kept in motion by cilia lining the tubes into which it is received. - Reproduction. All that is certainly known upon this subject is that the rotifera multiply by true ova, and never by gemming, budding, or spontaneous splitting, like the polygastrica. Until recently they were generally supposed to be hermaphrodite, but some late observers believe them to be unisexual. Ovaries are made out without difficulty, and in the vast majority of individuals; but spermatozoa have been found in only a very few, perhaps only one species. If males exist as a separate sex, they are probably only developed at one period of the year, and their term of existence is very short.

This is rendered probable by a very curious observation made by Mr. Dalrymple. He found in one genus male individuals, that possessed neither mandibles, nor alimentary canal, nor glands. The only apparatus that was fully developed was the generative. The animal was in fact a mere male genital system, endowed with power of independent existence, though that existence must have been of very short duration. The transparency of the tissues enables us to trace very satisfactorily the formation and progress of the ova. Their growth is very rapid, and they are in some genera extruded from the ovary two or three hours after their germ is first detected, and hatch in less than half a day. In other families the eggs remain in the ovary or cloaca, and are there hatched, the young being born alive. From the transparency of all the tissues, it is often possible to trace the form, and to a certain extent make out the details of the structure of the young animal while it is yet in the body of the parent.