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 In the short ecthorea of Sagartia the attenuated portion is obsolete.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.

The next form of these organs is the Tangled Cnida. This form is very generally distributed, and is mingled with the former in the various tissues. In the genus Sagartia, however, it is by far the rarer form, while in Actinia and Anthea it seems to be the only one.

The pretty little Corynactis viridis is the best species that I am acquainted with for studying this kind of cnida- . A fragment of its craspeda I have here ready for your observation, prepared exactly like that of C. Smithii. The figure of the cnidce is nearly that of a perfect oval, but a little flattened in one aspect, about -^o^h °^ an inch in the longer, and y^th in the shorter diameter. Their size, therefore, makes them peculiarly suitable for observations on the structure and functions of these curious organs. Within the cavity lies a thread (ecthoreum) of great length and tenuity, coiled up in some instances with an approach to regularity, but much more commonly in loose contortions, like an end of thread rudely rolled into a bundle with the fingers.

The armour of this kind does not differ essentially 2 B from that already described. It is true I have detected it only in Corynactis, where the short ectkoreum of the Tangled Cnida is surrounded throughout its length by a barbed strebla of three bands. The barbs are visible, under very favourable conditions for observation, even while the tangled wire remains inclosed in the cnida, but their optical expression is that of serratures of the walls, without the least appearance of a screw. This, I say, is the only species in which I have actually seen the armature of the ectkoreum in this kind of cnidce ; but I infer its existence from analogy in other species, where the conditions that can be recognised agree with those in this, though the excessive attenuation of the parts precludes actual observation of the structure in question. Spiral Cnidse constitute the third form.* In a few species, as Sagartia parasitica, Tealia crassicor/iis, and CeriantJms membranaceus, I have found very elongated fusiform cnida 1 , which seem composed of a slender cylindrical thread, coiled into a very close and regular spiral. In some cases the extremities are obtuse, but in others, as in T. crassicornis, an

CNIDA OF CORYXACTIS. example of which I now show you, the posterior extremity runs off to a finely-attenuated point, the whole of the spire visible even to the last ; the whole bearing no small resemblance to a multispiral shell, such as one of the Cerithiadce or Twrritelladae. The ecthoreum is discharged reluctantly from this form, and I have never seen an example in which

Dr. Karl Mobiiss (Abhandl. Naturw. Yer. z. Hamburg, 1866), considers this to be only an immature condition.— P. H. G., 1884.

sea-anemones: their weapons. the Mrhole had been run off. So exceedingly subtle are the walls of the cnidce, that it was not until after many observations that I detected them ; in an example from T. crassicornis, which had discharged about half of the wire, I could not see the slightest sign of armature on the ecthoreum. So far as my investigations go, these Spiral Cnidse are confined to the walls of the tentacles, in which, however, they are the dominant form.*

Such, then, are the form and armour of these organs. But I have not yet done with them. The emission of the wire, strange to say, is a process of distinct evolution, or expansion from within, from beginning to end. The ecthoreum is not a solid, but a tubular, prolongation of the walls of the cnida, turned-in, in its first condition ,. like the finger of a glove drawn inwards. Of this fact you may convince yourself by a careful watching of the phenomena before you. Many of the ecthorea from the tangled cnidce now under your eye run out, not in a direct line, but in a spiral direction. Select one of these, and you w T ill perceive that each bend of the spire is made, and stereotyped, so to speak, in succession, while the tips go on lengthening ; the tip alone progresses, the whole of the portion actually discharged remaining perfectly fixed ; which could not be on any other supposition than that of evolution.

In the discharge of the chambered kind (to revert to those which we were just now examining) we saw the ventricose basal part first appear ; the lower barbs flew out before the upper ones, and all were fully expanded before the attenuated portion began to lengthen. This, again, is consistent only with the fact of the evolution of the whole. On several occasions of observation on the chambered cnidce of C. Smithii, I have actually seen the unevolved portion of the ecthoreum running out through

These details, with many others, are given in my Hist, of Brit. Sea-anemones and Corals, Introd., xxii—xl. o P o the centre of the evolved ventricose portion. But perhaps the most instructive and convincing example of all was the following. One of the large tangled cnidce of Cori/nactis viridis had shot about half of its wire with rapidity, when a kind of twist, or "kink," occurred against the nipple of the cnida, whereby the process was suddenly arrested. The projectile force, however, continuing, caused the impediment to yield, and minute portions of the thread flew out piecemeal, by fits and starts. By turning the stage-screw, I brought the extremity of the discharged portion into view, and saw it slowly evolving, a little at a time. Turning back to the cnida, I saw the kink gradually give way, and the whole of the tangled wire quickly flew out through the nipple. I once more moved the stage, following up the ecthoreum, and presently found the true extremity, and a large portion of the wire, still inverted ; slowly evolving, indeed, but very distinct throughout its whole course, within the walls of the evolved portion.

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