the form of two hanging lobes or ears, which are richly clothed with vibrating cilia, and are powerful auxiliaries to swimming. Strong circular currents are formed around each ear, while the animal rests on its toes. As it swims, it is almost continually contracting the enormous head with a sort of spasmodic twitching, which brings the ear-lobes momentarily to point forward, and alternately, and almost as quickly, recovering itself. The conical lower parts of the body are also frequently contracted in various degrees, and drawn up to either side in the most irregular manner. These movements

are made by the various muscle-bands and threads, which exist in great number, and are discerned to much advantage, owing to the crystalline clearness of the tissues. This brilliant transparency is highly characteristic of the tribe, and, combined with their elegant form and sprightly motions, makes them rank among the most charming representatives of the class.

Just below the centre of the frontal crest there glows a very large oval eye of the richest ruby-crimson, through which the transmitted light is strongly refracted, as it would be by a globe of glass, showing that it is furnished with a true lens. This eye rests on a great mass of brain-substance, which when viewed from behind is seen to be much extended laterally. On each side of the eye are some large irregularly globose knobs of clear substance, from which descend two thick pieces, which meet at an angle near the middle of the body in the form of a massive V. From the summit of the frontal crest every now and then the animal protrudes two pairs of dark tooth-like points, curved towards each other, which open and close with a sudden snapping action, and are instantly withdrawn below the crest, where they can scarcely ever be discerned. For a long time I watched the recurrence of this motion, and marked the momentary protrusion of the jawteeth with the greatest interest, but with a most tantalizing impotence to solve their mystery. So totally unlike they seemed to the mouth apparatus in any other genus, that I longed for years to understand them. I saw by certain accompanying twitchings in the globose heads of the V-shaped body, that the protruded points were intimately connected with them; and I felt sure that if I could satisfactorily resolve them, I should acquire a flood of light on the as yet unintelligible structure of the mouth in the whole class. At length I thoroughly mastered it, and saw that it was by no means an abnormal apparatus, but essentially identical with the parts of what had been called the "gizzard" in the ROTIFERA generally. "in The two branches of the V are the mallei, or upper jaws, invested with thick masses of muscle: the hand, or "fork,"

consists of a single finger, the tip of which as a sharp curved point projects from the investing muscle, which here becomes globose. A delicately subtle rod which rises between the limbs of the V is the fulcrum of the lower jaws, seen at some distance forward, whose blades curve backward towards the beholder (as he looks at it dorsally); the whole being well imitated by the figure of the garden-shears. The blades, however, are invested with thick globose masses of muscle, out of which the tips project, as the inferior pair of snapping points. It is the massive character of the muscular investments which during life so disguises the form of the apparatus; and this massiveness, as well as the inordinate size of the whole structure, amounting to fully one-third of the entire bulk of the animal, indicates a predatory power which is by no means inconsistent with the impression of ferocity conveyed by the vigorous, sudden and frequently-repeated snappings, even when there seems nothing present to snap at.

I have mentioned above the name of the genus Polyarthra. It is a member of this order, but with peculiarities which isolate it from all others of the class. This, too, is a very difficult subject to make out, and it requires a long-continued study of the species, pursued through a great multitude of individuals, and a large acquaintance with other forms for comparison, well to understand its internal organization. This difficulty is augmented by its swift motion, its irregular fitful leaps, and its extreme transparency, combined with high refractile power.

Its form is that of a crystalline cup, somewhat like the body of a wine-glass without any foot, but bearing many thin, sword-shaped processes, which proceeding from the breast commonly lie flat on each side down the body, the points projecting below. These are evidently stiff and highly elastic, and their use is manifest to any one who sees the creature in active motion. It swims with a rapid gliding progress head foremost; but at almost every moment it makes a sudden forcible jerk or leap backwards, or to one side; and that so quickly, that the eye often cannot follow it in the transition. The organs by which these jumps are effected are the long breast-spines, which are suddenly thrown out in various directions, and they may frequently be seen extended the moment after a leap, before they reassume their normal position. When we consider that the creature is jerked often four or five times its own length through so dense a fluid, we shall perceive how strong the muscular action must be which moves the lever-like spines. The creature is thrown irregularly, often with the side foremost, or the back, or made to perform a somersault in the act. It is probably a sensitiveness to

danger or annoyance that prompts these violent leaps; at least it frequently performs them after a momentary examination of any floating matter with which its course brings it into

contact.

This very remarkable animal deviates in several particulars from the rest of the order, and yet I think its affinities are with them. It has no semblance of a foot, and the investiture of the body forms a stiff shell (lorica), as we find in the following order. Yet this is very peculiar. The back appears to me to be covered with longitudinal plates, divided from each other, and crossed by very delicate, close-set lines (stria). These plates appear to extend round to the ventral surface, protecting about one-third of the width on each side, and rising into a strong lateral point. The central third is occupied with membrane, which along the breast between the lateral points is rigid and shelly, but becomes quite flexible and skinlike towards the hind extremity.

The sword-like fins are exceedingly curious and quite unique; for though a genus named Triarthra is furnished with three long spines, by which it performs leaps, yet the structure of these organs is very different from that of the swords of Polyarthra, and the animal's affinities are with another order. These appendages are exquisite tests of the powers of the microscope, as well as of the skill of the observer. They are twelve in number, arranged in groups of three each, one group being set on each side of the dorsal, and one on each side of the ventral aspect, near the base of the projecting points of the breast. They are jointed, quite independently, on rounded shelly knobs, and are moved by strong muscles. Under graduated pressure I have brought out the four series of knobs, and the twelve fins with perfect distinctness, and have proved that the connection is that of true proper joints, like those of the limbs of insects. This existence of true joints, which is found in several other species of ROTIFERA, is one of the characters, on which I rely in maintaining, contrary to the opinion of Mr. Huxley and other excellent naturalists, the high place of the class in the natural system.

The fins are what botanists would call linear-lanceolate; very thin, with a midrib running throughout, acutely (in one case that I observed, rather abruptly) pointed, distinctly toothed along each edge, the teeth pointing forward. Their texture is excessively delicate, and shows oblique lines running to the notches; yet they are firm and highly elastic.

The female carries its eggs, after they are discharged, attached to her body for some time; another particular in which the species agrees with the Brachionida. The young student must, however, be on his guard against a very natural

mistake into which I myself fell in my early study of these forms. Observing that many specimens of this species had little green egg-shaped objects adhering to their bodies, and knowing the egg-bearing habit of the species, I took for granted that these were the ova. At length, however, a specimen occurred with one or two manifest eggs, of much larger dimensions, and perfectly agreeing in form, colour, appearance, and proportions with the eggs of other ROTIFERA, adhering to the body, while the little green objects were there also. At length some of these latter, detaching themselves, swam away by means of their own cilia, and I perceived that they were parasitic INFUSORIA, of the genus Colacium, which ordinarily infest this species, as well as ENTOMOSTRACA. Thus maturer observation corrects the errors of inexperience.

One of the most remarkable forms of this order is the genus Asplanchna, so named because of a most strange and quite unparalleled deficiency in its organization. Furnished with a formidable pair of nipping jaws, a long gullet, a capacious lobed stomach, and a distinct pair of pancreatic glands, and being highly predatory and carnivorous, this animal has not the slightest vestige of an intestinal canal, the digestive stomach hanging like a globe in the centre of the capacious body-cavity, tied to various points of the walls by long elastic slender muscle-threads, which allow it to sway about. From the great comparative size of the animals-for they are among the very largest of ROTIFERA,-from the brilliant transparency of the skin, and from the smallness and paucity of the viscera in proportion to the capacity of the ample body, the organs can be determined and examined with unusual facility; so that I know of none that present so favourable opportunities to the student who wishes to demonstrate the anatomy and physiology of this charmingly crystalline tribe of animals.

It is interesting to remark that the ovary in this genus is of the ordinary structure, though small. The young, however, are hatched, one at a time, within the body of the parent; and an exceedingly curious and interesting spectacle is presented by the young animal, perfectly formed, with its eyes, its jaws, and all its other organs readily identifiable, occupying one-third of the volume of the parent, as it lies horizontally across the blunt sac-like extremity, occasionally working its jaws, and moving its viscera to and fro by means of its muscle-threads; the double integument of the parent and offspring no more interfering with distinct vision than two thin films of glass. There is an outlet for the young at the period of birth, but it has no connection with the alimentary canal.

In some of the species not the slightest trace of a foot is

present, but in others there is a very small one, with two excessively minute toes.

Of the habits of the animals of this order, perhaps the most interesting examples are those afforded by two species of Notommata N. petromyzon and N. parasita, which live-the former occasionally, the latter habitually-in the majestic rolling spheres which bear the name of Volvox globator. In an interesting paper by the Editor of this REVIEW, the appearance and the salient points of the history of this remarkable organism, once considered an indubitable compound animal, now by most naturalists referred to the vegetable kingdom,-are well given.* Within the beautiful green spheres of the Volvox, N.parasita may be frequently seen comfortably lodged, whence Ehrenberg gave it the appropriate designation of "the Pirate." On examining several Volvoces, even with a pocket lens, we may frequently detect such as are thus tenanted by their containing one or more spots, differing from the young clusters in form and colour. Each of these spots is found to be a Notommata, snugly ensconced within the globe, in the spacious area of which it lives at ease, and swims to and fro like a gold-fish in a glass vase. We see it for the most part, however, clinging to the inner surface of the circumference, engaged in devouring the green monads with which its gelatinous expanse is studded, or else eating away the embryo clusters. The action of the protruded jaws of the Notommata, as it eagerly nibbles away at the monads, is very energetic; but its progress is not commensurate with its vehemence. I have not been able to see one actually detached and swallowed. Frequently two or more Notommata are found in the same Volvox, and I have seen as many as four, with an egg besides. They do not interfere with each other, but each pursues his own avocation.

It is chiefly the smaller Volvoces that contain the parasite, especially those which have the embryos very immature. I have not seen any loose in a large Volvox; but in embryos almost grown, and nearly ready for escape, I have several times seen a Notommata. If we wonder at this Rotiferon obtaining access to the interior of the sphere, we wonder still more at seeing even the young spheres, while yet within the mother sphere, thus tenanted, especially as the circumference both of mother and daughter appears unbroken. On careful examination, however, the parent Volvox shows places where the monads are interrupted; and I have thought that I could discover, in each case of a tenanted embryo, that its circumference had a ragged point.

The Volvox does not appear to suffer from the depredations

* "Popular Science Review," vol. i. p. 57.