SWOBD-BEAEEE.

of the mallei. The rami are very broad, somewhat square at their base, flat, but much arched longitudinally. They open and shut vigorously, with a snapping action, but are

not protruded from the front ; their Avhole interior edges come into contact. The mallei are simple, slender, bent rods, apparently without distinct articulation. During life they are thick and irregular in outline, owing to their being invested with dense muscles; as is the whole upper portion of the mastax. These muscles conceal or disguise the form and action of the parts during life ; but the introduction of a drop of solution of potash into the water instantly dissolves away the fleshy parts, leaving the solid organs, or those composed of chitine, beautifully clear, and fit for observation. Without this aid it would be impossible to resolve the structure of these minute animals.

The little Sword-bearer, like the Brachionus, carries its eggs attached to the hinder part of its body, for some time after they are discharged; the minute green oval bodies that you see sticking to the side of this specimen, are not, however, eggs, but parasitic animalcules (Colacium vesiculosum), which very frequently infest this species, adhering to various points of the shell, and even to the sword-fins.

What I have nowto submit for your examination is one of the rarest species of the class, and certainly not the least singular in its form. It is the tripod Wheel-bearer (Actinurus Keptunius). When fully extended, its length exceeds that of almost every other species, for it reaches about one-twentieth of an inch ; but its extreme threadlike slenderness precludes the unassisted eye from taking cognizance of it, as its thickness, even when greatest, is not more than one six-hundredth of an inch.

From this excessive length and tenuity, the appearance of the creature is very remarkable. It may be likened to a cylindrical tube, out of which protrude a great number of draw-tubes from both extremities, principally the posterior one. Those in front terminate in an oval proboscis, which, having a sort of ringer at its extremity, and two eyes, with an antennal tube projecting obliquely backwards, presents, when viewed laterally, a strong resemblance to the head of a rabbit, the antenna representing the ears. In front, and just below this head-like proboscis, is a double swelling, containing the rotatory organs, which are small and seldom unfolded. The eyes are deep black ; probably, as in the last example, a red of great intensity. When the head is withdrawn, the integument is very clearly seen to be turned inwards. The body consists of one long cylindrical tube, which receives three or four short joints to complete the abdomen ; at the dorsal point of the extremity of the last of these is the cloaca ; at this part the diameter is already very much attenuated ; but there are eight or nine more joints which constitute the foot, and these are of extreme slenderness. Towards the extremity, two processes are given off behind, each consisting of a club- TBIPOD WHEEL-BEAREB. shaped piece, with a slender bristle at the tip. The foot terminates in three long, slender, cylindrical, divergent toes, which are flexible, and commonly bent outward ; they are equal in thickness, and truncate. These are often retracted in various degrees, even when the foot is otherwise extended.

Owing to the slenderness of the body, the viscera are greatly elongated. The mastax, as usual in this family, consists of two hemispheres (each bearing two teeth, set transversely, but converging to the centre) ; it is situated at a considerable distance from the wheels, and is reached by a long buccal funnel. The digestive canal is a long sac, apparently undivided ; it originates directly from the mastax, with, I think, two small basal glands ; its posterior extremity becomes gradually tapered to the cloaca. In the specimen we are examining, a small quantity of

f?ecal matter of a yellowish-brown colour is collected in two small masses, near the extremity. Along the under side runs the ovary, which in this specimen contains two long oval eggs in advanced development; from their transparent brightness, I suspect the young are produced before birth. I think I can detect a contractile bladder, but am not certain.

The dorsal region of the trunk is marked with strong rugged lines running longitudinally; these look like corrugations of the integument, but I incline to think them the strongly developed muscles for the retractation of the foot. Muscles are seen running through the joints of the foot, until they can no longer be traced, from their tenuity. The viscera can be detected with difficulty, partly owing to the longitudinal muscles, which are so strong and close, and partly from the incessant contraction and elongation of the parts, which drive the internal organs hither and thither. It refuses, you see, to swallow carmine, which might have assisted us.

This singular animal is lively in its motions, especially in the protrusion and retractation of the extremities. These are constantly alternating, and a very curious sight it is to see the immense length of foot suddenly thrust forth from the body, in which it has been completely hidden, the starting out of the horizontal processes, and the diverging of the long toes, as these are successively uncovered. The latter do not seem to be often used as instruments of prehension or adhesion. Indeed, the animal does not appear very much given to change of place, but lies in the water, alternately contracting and elongating. Frequently, as the foot is thrust out, the body is made to bend forward so as to form a right angle (see the engraving, in which the animal is thus represented at a ; b represents it when the head is rotating, but the foot is almost wholly withdrawn within the body ; in which state the resemblance to a telescope, or to a nest of glass tubes, is striking).

The last specimen of this class of tiny favourites that I shall show you is one of more than ordinary beauty. It is the Two-lipped Tube-wheel of the Hornwort (Limnias ceratophylli). Hitherto we have seen such examples as have the power of freely swimming from place to place at pleasure ; but there is a considerable group, of which this is a member, which are permanently stationary, being fixed for life to the leaves or stems of the vegetation that grows under water. The stiff and spinous whorls of the Hornwort (Ceratophyllum demersum), that grows commonly in sluggish streams and pasture-pools, is a favourite resort of the species, but it is not confined to any one plant. Here, for instance, it has chosen as the site of its residence the much-cleft leaves of the Water Crowfoot (Ranunculus aquatilis) ; those leaves, I mean, which, growing wholly under the water, are divided into a multitude of slender finger-like filaments, so different from those which float on the surface, and which are merely notched.

You can readily find the Tube-wheels by the aid of a pocket lens, and even with the naked eye when you have seen one or two. By holding up this phial, in which a little plant of the Crowfoot is growing, and searching, with the lens, the window being in front of you, the filaments, one by one, you will readily perceive, here and there, little shining objects standing up or projecting in various directions from the surface of the leaves. The colony is rather numerous in this case, and we shall have no difficulty in selecting our specimens.

On this filament, which I have seized with the tips of a pair of pliers, I can see at least half-a-dozen of the little parasites. This, then, I will nip off from the plant, and put it with its tiny population into the livebox. Here it is ready for examination.

Several of the animals are in the field of view ; but we will look at one at a time. A long narrow tube, slightly widening at the mouth, is affixed by the lower extremity to the slender filaments of water-grass, crowfoot, &c. It is about one fifty-fifth of an inch in length, pellucid, but tinged with brownish yellow. It appears to be of a gelatinous texture, and is covered with foreign substances, such as decaying animal or vegetable matters, which adhere to its surface. From the mouth of the tube protrudes a transparent colourless animal, the head of which is rounded, with the extremity pursed up. Suddenly it unfolds its flower-like wheel, which consists of two broad nearly circular lobes united, the margin of which is set with strong cilia, much resembling those of the last species. Each cilium appears to be curved, and to be thickened at the middle—the optical expression of the ciliary wave ; and the effect of the rotation, as each seems to pursue its fellows around the circular course down the dividing sinuosity, up the opposite sides, and round the margin again, is very striking. The cilia at the front are interrupted between the lobes. In H the centre of each lobe is a broad plate,

TUBE-WHEEL. TWO-LIPPED surrounded by a bright ring, and crossed by radiating lines, which also extend towards the ciliated margin ; probably these are muscular filaments. The funnel is between the lobes, and leads by a short oesophagus, or gullet, to a bulbous transparent mastax, in which are seen jaws that w Tork on each other. Below this is a long capacious sac, without convolutions or constrictions, but apparently granular in its texture. The alimentary canal is bent upwards through the whole length, terminating in an orifice behind the rotatory organ ; for though I have not traced it when empty, I have seen

the faecal matter driven rapidly upward as through a canal, until the mass was discharged just behind the sinuous cleft. On our mixing carmine with the water, the effect is very striking; the particles, whirled round in two circular vortices, are poured in an accumulated torrent through the sinuosity, and over the elevation at the front of the head. We presently perceive a slender line of crimson passing down below the mastax, which indicates a slender stomach-tube there ; and, after a while, a little ball of the same pigment accumulates, and is seen resting a little lower down. This, then, indicates the form and position of the stomach; it must be a very slender canal, terminating in a small rounded bag, at about one-third of the distance from the mastax to the base of the tube. The lengthened sac which you see is the ovary, from which the eggs are discharged into the lower part of the case.

WHEELS OF TUBE-WHEEL.

The mouth needs a little explanation in detail. As you see it, you probably discern little resemblance in its parts to the same organ in Braclrionns, and yet essentially it is formed of the very same parts; and as it is very instructive to observe the modifications, in different animals, of a common model of any particular organ, it will be worth while to devote a few minutes' careful observation to this structure before us, especially as it is here seen with more than usual brilliancy and clearness.

The mastax, then, which you see in the centre of the

animal, just below the level of the beautiful flower-like wheels, consists as usual of three sub-globose lobes; one on each side appropriated to each malleus, and the third descending towards the ventral aspect, which envelops the incus. The mallei are more intimately united to the rami of the incus than in the former type, each uncus forming, with its ramus, a well-defined mass of muscle, inclosing the solid parts, and in form approaching that of a quadrant, or fourth part of a circle; two flat faces opposing and working on each other. Across the upper surface of the mass the uncus is stretched, as three longparallel fingers arched in their common direction, and imbedded in the muscular substance; their points just reaching the opposite face of the ramus, and meeting the points of the opposite uncus, when closed. The manubrium is much disguised, by being greatly dilated transversely, forming three bow-like loops of little solidity, to the chord of which the fingers are soldered, not articulated. The surface of the dense muscular mass disjDlays stria3 , or streaks, parallel to the fingers, and, as it were, continuing their number towards their dorsal extremity, becoming fainter till they are imperceptible. These stripe do not disappear when the muscular parts are dissolved by potash; and hence I infer the existence of a delicate investiture of solid substance similar to that of the teeth, ifec, inclosing the muscular mass.

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