The dimensions of the cinclides vary not only with the species, and probably also with the size of the individual, but with the state of the muscular contraction of the integument, as, also, I think, with the pleasure of the animal. In a small specimen of S. dianthus, I found the width of a cinclis, measured transversely, yfyth of an inch ; but that of another, in the same animal, was more than twice as great, viz., y^yth of an inch. This was on the thickened marginal ring, or parapet, which in this species surrounds the tentacles, where the cinclides are larger than elsewhere. Watching a specimen of S. nivea under the microscope, I saw a cinclis begin to open, and gradually expand till it was almost circular in outline, and -^^th of an inch in diameter. I slightly touched the animal, and in an instant it enlarged the aperture to y^th of an inch. In a specimen of & bellis, less than half-grown, I found the cinclides numerous, and sufficiently easy of detection, but rather less defined than in dianthus or nivea. They occurred at about every fourth intersept, three intersepts being blind for each perforate one; and about three or four in linear series, but not quite regularly in either of these respects. In this case they were about -~^th. of an inch in transverse diameter—a large size ; and I measured one which was even -g^th of an inch. By bringing the animal before the window, I could discern the light through the tiny orifices with my naked eye.

From several good observations, and especially from one on a cinclis, widely opened, that happened to be close to the edge of the parapet of a dianthus, I perceived that the passage is not absolutely open, at least in ordinary; but that an exceedingly thin film lies across it. By delicate focussing, I have detected repeatedly, in different degrees of expansion, and even at the widest, the granulations of a membrane of extreme tenuity, and one or two scattered cnidw, across the bright interval. On another occasion, in the case of a cinclis at the edge of the parapet, a position singularly favourable for observation, I saw that this subtle film was gradually pushed out until it assumed the form of a hemispherical bladder, in which state it remained as long as I looked at it. At the same time the outline of the cinclis itself was sharp and clear, when brought into focus further in. The film, whatever it be, is superficial, and does not appear to be a portion of the integument proper. I take it to be a film of mucus (composed of deorganised epithelial cells), which is constantly in process of being sloughed from all the superficial tissues in this tribe of animals, and which continues tenaciously to invest their bodies, until, corrugated by the successive contractions of the animal, it is washed away by the motions of the waves. As, however, one film is no sooner removed than another begins to form, one would always expect external pores so minute as these to be veiled by a mucus-film in seasons of rest.

The pressure of this film is sufficient evidence that the cinclides are not excretory orifices for the outflow of the respired water in the manner of the discharging siphon in the Bivalve Mollusca :— at least that no current constantly, or even ordinarily, passes through them. I have watched them continuously for periods sufficient to detect such discharge if it were periodic. On one occasion (viz., that in which the film was protruded like a blown bladder) a minute Infusorial animalcule chanced to pass across, close to the surface of the film : this would have been a decisive test of the existence of a ciliary current ; but not the slightest deviation in the little atom's course could be detected.

That the cinclides are the special orifices through which those missile weapons, the acontia, are shot and recovered, rests not merely on the probability that arises from the co-existence of the two series of facts I have above recorded, but upon actual observation. In a rather large S. dianthus, somewhat distended, placed in a glass vessel between my eye and the sun, I saw, with great distinctness, by the aid of a pocket-lens, many acontia protruded from the cinclides, and many more of the latter widely open. The acontia, in some cases, did not so accurately fill the orifice but that a line of bright light (or of darkness, according as the sun was exactly opposite or not) was seen partially bordering the issue of tinthread, while the thickened rim of the cinclis surrounded all.

The appearance of the orifices whence the acontia issued was that of a tubercle or wart, and the same appearance I have repeatedly marked in examples observed on the stage of the microscope ; namely, that of a perforate pimple, or short columnar tube. This was clearly manifest when the animal, slowly swaying to and fro, brought the sides of the cinclis into partial perspective.

On another occasion I witnessed the actual issue of the acontia from the cinclides. I was watching, under a low power of the microscope, a specimen of a S. nivea, while, by touching its body rudely, I provoked it to emit its missile filaments. Presently they burst out with force, not all at once, but some here and there, then more, and yet more, on the repeated contractions of the corrugating walls of the body. Occasionally, the free extremity of a filament would appear, but more frequently the bight ofa bent one ; and very often I saw two, arid even three, issue from the same cinclis. The successive contractions of the animal under irritation, caused the acontia already protruded to lengthen with each fresh impetus, the bights still streaming out in long loops, till perhaps the free end would be liberated, and it would be a loop no longer ; and sometimes a new thread would shoot from a cinclis, whence one or two long ones were stretching already : while, as often, the new-comers would force open new cinclides for themselves. The suddenness and explosive force with which they burst out, appeared to indicate a resistance which was at length overcome :—perhaps (in part at least) due to the epithelial film above mentioned, or to an actual epiderm,* which, though often ruptured, has ever, with the aptitude to heal common to these lowly structures, the power of quickly uniting again.

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.

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