It appeared to me manifest, from this and other similar observations, that no such arrangement exists as that which I had fancied—that a de6nite cinclis is assigned to a definite acontium, or pair of acontia ; and that the extremity of the latter is guided to the former, with unerring accuracy, by some internal mechanism, whenever the exercise of the defensive faculty is desired. What I judge to be the true state of the case is as follows : The acontia, fastened by one end to the septa, or the membranes which support them, lie, while at rest, irregularly coiled up along the narrow interseptal hollows. The outer walls of these hollows are pierced with the cinclides. When the animal is irritated, it immediately contracts : the water contained in the visceral cavity finds vent at these natural orifices, and the forcible currents carry with them the acontia, each through that cinclis which happens to lie nearest to it. The frequency with which a loop is forced out shows that the issue is the result of a merely mechanical action ; which is, however, not the less worthy of admiration because of the simplicity of the contrivance ; nor the less manifestly the result of Divine wisdom working to a given end by perfectly adequate means. The ejected acontia, loaded with their deadly cniclce in every part of their length, carry abroad their fatal powers not the less surely than if each had been provided with a proper tube leading from its free extremity to the nearest cinclis.

Curious as these contrivances are, there is yet much more to be told : these are preparatory and ancillary, as it were, to the elaborate mechanism by which the ultimate object of the whole provision is to be attained. The

The outer covering of the external surface of the body, commonly called the cuticle or scarf-skin.

acontium is but a reservoir for the weapons,—a kind of quiver for the arrows; and the cinclis is a provision for getting them ready for action : we have not yet looked at the arrows themselves.

They occur under three principal forms ; and for the investigation of these we shall find it convenient to have recourse to different species.

The first and most generally distributed form is the Chambered Cnida, as it is also the most elaborately organised. I know of no species in which it can be examined under so favourable circumstances as the pretty Madrepore (Caryophyllia Smithii) of our south-western coasts ; and, as I have several specimens of that species in my aquarium, subjects are at hand for our investigation. The clear tentacles are, as you perceive, crowned with opaque globular heads ; if I should nip off one of these heads, and flatten it by means of the compressorium, you would see it literally composed of cnidce, the ends of which project side by side, as close as they can be packed one against another. But still larger examples may be obtained from the craspeda. With a smart sudden blow I break the stony skeleton of the Madrepore in sunder—the flesh tearing apart also ; and thus I expose the interior of the living animal. A great number of pellucid ribbons are now seen, very much convoluted, which are named craspeda. These are almost composed of large cnidce. I remove with fine pliers a small fragment of one of these ribbons, and, placing

CNIDA OF it between the plates of the compressorium,

MADEEPOEE. flatten it gradually till the plates are brought

into as close contact as they can be. A high power now being put on, examine the organs in question.

You see a multitude of perfectly transparent, colourless vesicles, of a lengthened ovate figure, considerably larger at one end than at the other ; one of average dimensions measures in length ^^jth of an inch, and in greatest diameter 2 * th. In the larger (the anterior) moiety, passing longitudinally through its centre, is seen a slender chamber, fusiform or lozenge-form, about eVooth of an inch in its greatest transverse diameter, and tapering to a point at each extremity. The anterior point merges into the walls of the cnida at its extremity; while the posterior end, after having become attenuated like the anterior, dilates with a funnel-shaped mouth, in which the eye can clearly see a double infolding of the chamber-wall. After this double fold the structure proceeds as a very slender cord, which, passing back towards the anterior end of the capsule, winds loosely round and round the chamber, with some regularity at first, but becoming involved in contortions more and more intricate, as it fills up the posterior moiety of the cavity. The fusiform chamber appears to be marked on its inner surface with regularly recurring serrations, which are the optical expression of that peculiar armour to be described presently.

Under .the stimulus of pressure, when subjected, as now, to microscopical examination, and doubtless under nervous stimulus, subject to the control of the will, during the natural exercise of the animal's functions,—the c?iidce suddenly emit their contents with great force, in a regular and prescribed manner. It must not be supposed, however, that the pressure spoken of is the immediate mechanical cause of the emission : the contact of the glass-plates of the compressorium is never so absolute as to exert the least direct force upon the walls of the capsule itself ; but the disturbance produced by the compression of the surrounding tissues excite3 an irritability, which evidently resides in a very high degree in the interior of the cnidcp : and the projection of the contents is the result of a vital force.

In general, the eye can scarcely, or not at all, follow the lightning-like rapidity with which the chamber[and its twining thread are shot forth from the larger end of the cnida. But sometimes impediments delay the emission, or allow it to proceed only in a fitful manner, a minute ])ortion' ;at a time ; and, sometimes, from the resistance of friction (as against the glass plate of the compressorium), the elongation of the thread proceeds evenly, but so slowly as to be watched with the utmost ease ; and sometimes the process, which has reached a certain point normally, becomes from some cause arrested, and the contents of the cell remain permanently fixed in a transition state. Thus, a long-continued course of patient observation is pretty sure to present some fortuitous combinations, and abnormal conditions, which greatly elucidate phenomena, that normally seemed to defy investigation.

In watching any particular cnida, the moment of its emission may be predicted with tolerable accuracy, by the protrusion of a nipple-shaped wart from the anterior extremity. This is the base of the thread. The process of its protrusion is often slow and gradual, until it has attained a length about equal to twice its own diameter, when it suddenly yields, and the contents of the cnida dart forth. At this instant I have, in many instances, heard a distinct crack or crepitation, both in the examination of this species and of Sagartia parasitica.

When fully expelled, the thread or wire, which is distinguished by the term ecthoreum, is often twenty, thirty, or even forty times the length of the cnida; though in some species, as in most of the Sagartice, it frequently will not exceed one-and-a-half or two times the length of the cnida.

The ecthorea which are discharged by chambered cnida3 are invariably furnished with a peculiar armour. The l>asal portion, for a length equal to that of the cnida, or a little more, is distinctly swollen, but at the point indicated it becomes (often abruptly) attenuated, and runs on for the remainder of its length as an excessively slender wire of equal diameter throughout. In the short ecthorea of Sagartia the attenuated portion is obsolete.

It is chiefly upon this ventricose or swollen basal portion that the elaborate armour is seen, which is so characteristic of these remarkable organs. For around its exterior wind one or more spiral thickened bands, varying in different species as to their number, the number of volutions made by each, and the angle which the spiral forms with the axis of the ecthoreum. The whole spiral formed of these thickened bands is termed the screw, or strebla.

In the ecthorea emitted by chambered cnidce from the craspeda of Tealia crass-icornis, the screw is formed of a single band, having an inclination of 45° to the axis, and becoming invisible when it has made seven volutions. In those from the same organ in &. parasitica, we find TEALIA CXIDA CRASSrCORXIS OF the screw of two equidistant bands, (discharged) . each of which makes about six turns— twelve in all—having an inclination of 70° from the common axis. In those similarly placed in Caryophyllia Smithii [now under your observation], the strebla is composed, as you may perceive, of three turns—thirty equidistant bands, all— each of which makes about ten in with an inclination of about 40° from the axis. In every

case the spiral runs from the east towards the north, supposing the axis to point perpendicularly upwards.

Sometimes, especially after having been expelled for some time, the wall of the ecthoreumhecomes so attenuated as to be evanescent, while the strebla is still distinctly visible. An inexperienced observer would be liable, under such circumstances, to suppose that the screw, when formed of a single band, as in T. crassicornis, is itself the wire ; an error into which I had myself formerly fallen. An error of another kind I fell into, in supposing that the triple screw of the wire in C. Smiihii was a series of overlapping plates : the structure of the weapon is the same in all cases (with the variations in detail that I have just indicated) ; and the structure is, I am now well assured, a spiral thickened band running round the wall of the ecthoreum on its exterior surface. I have been able, when examining such large forms as these of Gorynactis viridis and Caryophyllia Smithii, with a power of 750 diameters, to follow the course of the screw, as it alternately approached and receded from the eye, by altering the focus of the object-glass, so as to bring each part successively into the sphere of vision.

These thickened spiral bands afford an insertion for a series of firm bristles, which appear to have a broad base and to taper to a point. Their length I cannot determinately indicate, but I have traced it to an extent which considerably exceeds the diameter of the ecthoreum. These barbed bristles are denominated pterygia.

The number of pterygia appears to vary within slight limits. As well as I have been able to make out, there are but eight in a single volution of the one-banded strebla in T. crassicornis ; while in the more complex screws of S. parasitica, Cor. viridis, and Car. Smithii, there appear to be twelve in each volution.

The barbs, when they first appear, invariablyproject in a diagonal direction from the ecthoreum; and sometimes they maintain this posture. But more commonly, either in an instant, or slowly and gradually, they assume a reverted direction.

From some delicate observations made with a very good light, I have reason to conclude that the strebla, and even the pterygia^ are continued on the attenuated portion of the ecthoreum, perhaps throughout its length. In Cor. viridis and Car. Smithii, I have succeeded in tracing them up a considerable distance. In the latter I saw the continuation of all these bands with their bristles; but, what was strange, the angle of inclination had become nearly twice as acute as before, being only 22° from the axis. The appearance of the attenuate portion, as also of the base of the ventricose part, is exactly that of a three-sided wire, twisted on itself ; the barbs projecting from the angles.

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