them as "animals with club-shaped bodies, able to contract themselves into a ball, and capable, when extended, of drawing in their anterior extremity, or protruding in its place a double lobe with cilia, presenting the aspect of two wheels in motion; terminating posteriorly by a manyjointed tail, the last joint carrying a pair of fleshy fingers or styles. Swimming by means of vibratile cilia, or crawling like leeches by alternately fixing the two extremities of the elongated body. Jaws stirrup-like. Two or more red eye points." We shall presently see the real nature of the jaws so described; but with the mention of the aspect of this family we conclude our brief review of some of the leading varieties of rotifer forms.

Let us now revert to the trochal disks and adjacent ciliary apparatus.

In the first place it may be generally remarked that the ciliary organs of rotifers are more complicated than they may appear at first sight, or than they are sometimes described. An immense difficulty in the way of understanding their real structure and action arises from the awkward perspective in which we usually see them. It is but rarely that we can look straight down upon them so as to see their ground plan at one glance. Ordinarily, we see an imperfect sectional view, as if we saw a perpendicular slice of a house exhibiting in section the front set of rooms, and with imperfect glimpses of other rooms behind. We have also to do with objects so minute, and with parts so transparent, that only the most careful focussing and illumination, conjoined with much thought, can enable us even approximately to ascertain the exact superposition of different structures and the way in which they cc-operate with each other. Mr. Huxley's paper on Lacinularia socialis, in the "Transactions of the Microscopical Society" for Dec. 1851, will give us valuable help in this matter. The trochal, or wheel-disk of the Lacinularia is, according to his description, "wide, and horse-shoe shaped. The edge of the disk has a considerable thickness, and presents two always distinct margins, an upper and a lower, of which the former is thicker and extends beyond the latter. The large cilia are entirely confined to the upper margin and seated upon it; they form a continuous horse-shoe shaped band, which, upon the oral side, passes entirely above the mouth. The lower margin is smaller, and less defined than the upper, its cilia are fine and small, not more than one-fourth the size of those of the upper margin. On the oral side this lower band of cilia forms a V-shaped loop. About the middle of this margin, on each side, there is a small prominence, from which a lateral ciliated arch runs upwards into the buccal cavity, and below becomes lost in the

VOL. XII.-NO. IV.

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cilia of the pharynx. The aperture of the mouth, therefore, lies between the upper and lower ciliary bands." A simple set of wheel-cilia, always acting in the same way, would throw their whirlpool currents in one direction, but the addition of other cilia may readily enable the direction of the currents formed by the wheel-cilia to be changed, so as to suit the varying wants of the creature. The precise modes in which the wheel-cilia and the subordinate cilia are arranged vary considerably, but in every case in which I have been able to examine the ciliary apparatus under favourable circumstances, and with powers of from 600 or 700 to 1000 or 1500, I have found the quantity of cilia much larger than lower powers indicated, and the functions of the cilia, or of some of them, more varied than is commonly described. In examining a tube-dwelling species last year-the Ecistes, which Mr. Davis brought before the Royal Microscopical Society, with Beck's one-twentieth object-glass and powers of from about 900 to 1500-the wheel-cilia appeared as dense tufts, and many cilia were noticed in a position to assist in directing the chief currents, and which were not concerned in producing them. This creature repeatedly bent its head on one side, and, if I may use such a phrase, "licked" the matter adhering to a thread of conferva, with long cilia, apparently belonging to the wreath or to one of its rows. This licking action sometimes went on simultaneously with the rotatory movement.

Placing a single Conochilus under a compressorium, and viewing it with a power of 1000, an inner row of cilia was seen to be engaged in conveying particles down the buccal canal.

In the common rotifer, when the creature is extended and its wheel organs retracted, it is common to see a few cilia moving about its mouth. It can alter the form of its anterior extremity to a considerable extent, and appears to have full control of the cilia it occasionally throws out.

With respect to the cause of ciliary movement, nothing positive is known, but some cilia appears to be always in motion, while others belong to a voluntary or quasi-voluntary system. The wheel appearance results from the rythmical motion of various cilia moving in succession. When the movements are not too fast, and a sufficiently high power is displayed, it appears that each cilium is thrown into progressing and retreating curves, just as a long thin wand of whalebone may be thrown into wave motions by the wrist. When a breeze moves over a field of corn the aspect of advancing or retreating waves is produced by the alternate bendings and straightenings of the corn stalks, and a somewhat similar set of movements progressing round a ring, gives rise to the appearance of rotation so strikingly shown by the common rotifer and its allies.

The transition from strongly vibratory cilia to the more quiescent cilia belonging to the Floscularia and Stephanoceros seems abrupt, if we turn immediately from active wheel-cilia to the latter forms, but an attentive examination of rotifiers will often reveal cilia acting so gently as to bridge over the difference of the two kinds, and probably if the long quiescent cilia were strongly agitated by any force applied to their proximate extremity, that is, to the end which is inserted in the animal, they might execute the wave motions of the other sort. The particles composing wheel-cilia must be in a state of strong tension, and this would seem to result, not from a mere mechanical arrangement, as in a piece of whalebone or a steel spring, but from a cause which ceases with the life of the creature. As soon as a Conochilus, for example, is dead, its ciliary wreath falls together, in a comparatively soft-looking and inert mass. On the other hand, cilia less distinctively associated with voluntary motion continue their action when torn away from the organism of which they formed a part.

After the wheel-organs of rotifers, their so-called "gizzards" are their most striking features. These remarkable structures were very imperfectly known until Mr. Gosse communicated to the Royal Society his admirable papers on the subject. Mr. Gosse employed a solution of caustic potash to dissolvewhich it does instantly-the soft flesh of the rotifers, and then was able to obtain the manducatory apparatus or gizzard in a separate form. By managing the screws of his compressorium so

1 a

d

4

k

C

2

3

FIGS. 1, 2, and 3.-Mastax of Brachionus urceolaris (Gosse).

FIG. 4.-Mastax of Notommata clavulata (Gosse).

as to create water currents, he caused those organs to revolve in various directions, and with wonderful patience and skill made out details of structure and principles of action which had escaped previous observers. Mr. Gosse regards the gizzard as a true mouth, and he calls the muscular bulb containing the biting and grinding machinery a mastax. In Brachionus urceolaris and some of the Hydatina he finds the ap

paratus most highly developed, and we may select a Brachion as our leading example. The preceding figure (1) of the mastax of Brachionus urceolaris, is copied from Mr. Gosse, and it will be more easily understood if we placed near it another sketch, Fig. 4, copied from the same writer, of the gizzard of the Notommata clavulata.

In the Brachion mastax we see two, to use Mr. Gosse's words, "geniculate organs," b, which he likens to hammers working on an anvil, and names mallei, and a third, f, still more complex, the incus (an anvil). These three pieces are not arranged in the same plane: for the mallei approach each other a little dorsally, while the incus is placed on the ventral side of the centre, its stem pointing considerably towards the same side. . . . Each malleus consists of two principal portions articulated with each other by a powerful joint, which seems to be ginglymate in its character, admitting of motion in one plane only, . . . the inferior portion of the malleus, which I call the manubrium (handle), c, is an irregularly curved piece, shaped somewhat like the scapula of a mammal, knotted on its broad and flattened at its lower or fine end, where also it is twisted on one side, ridges run down it on both the exterior and interior surfaces, the head is obliquely truncate, and it is this oblique surface, d, that is joined to the superior portion, e, which, from its prevalent form in the other genera, rather than in this, I call the uncus (hook). . . . It consists of five or six finger-like processes (Figs. 2 and 3), set parallel to each other, and separated by narrow interspaces, which appear to be occupied by a thin membrane. . . The incus, f, consists of distinct articulated portions. The principal are two stout rami, g, resting on what appears to be a slender pedicel (fulcrum b), but viewed laterally, the fulcrum is seen to be a thin plate to the edge of which the rami are jointed, so that they open and shut like a pair of shears. Each ramus is a thick somewhat trigonal piece with the outer side rounded, the upper side hollowed, and the inner side flat, and in contact with the corresponding face of its fellow, in a state of repose. The uncus of each malleus falls respectively into the concavity of each ramus, and is fastened to it by a stout triangular muscle." The two unci alternately recede and approach, and at the same time the motion is complicated by a twist of the manubria. "The incus also has considerable motion, sometimes the fulcrum is elevated, and the rami depressed, so that the former is invisible; the rami open and shut with the working of the mallei, being fastened to them by the strong triangular muscle above mentioned. . . . It is also evident that they have a motion of separating and closing independent of the mallei, though this is comparatively limited in extent, and not very often exercised. Again, when substances are brought into

contact with the jaws, which for any reason are not acceptable, they are thrown up through the buccal funnel by a peculiar scoop-like action of the unci, which is very curious to witness."

A simplification of this description will obviously apply to Notommata clavulata, and to the jaws of the Asplanchna Brightwellii, shown in the plate; but it should be remarked that Mr. Gosse shows how strongly the jaws of the Asplanchna resemble the incus portion of the more complicated jaws of Synchata tremula and Diglena forcipata, the mallei in Asplanchna being scarcely visible, and possessing no important function. Mr. Gosse's paper must be consulted to understand how the various parts are modified, or become obsolete in different genera; but it is advisable here to cite his popular explanation of the state of the organ in the common rotifer. He says, "" Suppose an apple to be divided longitudinally, having the stalk attached to one half. Let this now be split longitudinally, so far as the stalk, but not actually separating any portion from it. Draw the two portions apart, and lay them down on their rounded surfaces. They now represent the quadrantic masses [composed of soldering together the mallei and incus], the stalk being the fulcrum, and the upper surfaces being crossed by the teeth. By the contraction of the muscles of which they are composed, the two segments are made to turn on their long axes until the points of the teeth are brought into contact, and the toothed surfaces rise and approach each other."

In the Floscules, Mr. Gosse finds the mastax, or bulb, wanting, "the dental apparatus, which is very small, springing from the paries (wall) of the stomach just below the second diaphragm."

The preceding quotations will serve as an introduction to the study of this, the so-called gizzard of rotifers; but it is important to state in connection with Mr. Gosse's opinion, that these organs are jaws, that he has witnessed their protrusion in Furcularia, Pleurotrocha, Taphrocampa, Notommata clavulata, N. aurita, N. petromyzon, N. parasitica, Plagiognatha, Scaridium, Synchota, Polyarthra, Diglena, Asplanchna, Mastigocerca, Monocerca, Salpina, Monostyla, and Anuraa. As some of these are common species, Mr. Gosse's statements are easily verified by any student.

The stomachs and intestines of rotifers are richly ciliated, and near the stomachs may usually be seen bodies of considerable size, which appear to be organs secreting some fluid necessary to the process of digestion. In Asplanchna, the very curious circumstance occurs of a creature so highly organized being destitute of an intestine, or an anal orifice. The undigested portions of its food being cast out through the gullet and