texture of the plates or jaws resembles that of the shell in every respect, save that the network is more open; but that of the teeth is much more compact. The latter have been described by Mr. Quekett as consisting of a substance not altogether unlike the "dentine" of the teeth of higher animals, save that the tubuli, though sometimes parallel, usually have more of a reticulated arrangement, and sometimes dilate into irregular "lacunæ" or spaces excavated in the hard substance.' The Author is not prepared to speak with confidence on this point; but he is disposed to think that the structure of the teeth is essentially the same as that of the shell, save in the interspaces of the network being much narrower; and that the appearance of tubuli (in which Mr. Quekett has not been able to make out distinct walls) is due merely to the elongation of these interspaces.

FIG. 239.

315. The calcareous plates which form the less compact skeletons of the Asteriada (star-fish and their allies) and of the Ophiurida (sandstars and brittle-stars), have the same texture as those of the shell of Echinus. And this presents itself, too, in the spines or prickles of their surface, when these (as in the large Goniaster equestris) are large enough to be furnished with a calcareous framework, and are not mere projections of the horny integument. An example of this kind, furnished by the Astrophyton (better known as the Euryale), is represented in Fig. 239. The spines with which the arms of the species of Ophiocoma (brittle star) are beset, are often remarkable for their beauty of conformation; that of O. rosula, one of the most common kinds, might serve (as Prof. E. Forbes justly remarked) in point of lightness and beauty, as a model for the spire of a cathedral.

Calcareous plate and claw of Astrophyton (Euryale).

316. The calcareous skeleton is very highly developed in the Crinoidea; their stems and branches being made up of a calcareous network, closely resembling that of the shell of the Echinus. This is extremely well seen, not only in the recent Pentacrinus Caput Medusa, a somewhat rare animal of the West Indian seas, but also in a large proportion of the fossil Crinoidea, whose remains are so abundant in many of the older geological formations; for, notwithstanding that these bodies have been penetrated in the act of fossilization by a mineral infiltration, which seems to have substituted itself for the original fabric (a regularly crystalline cleavage being commonly found to exist in the fossil stems of Encrinites, &c., as in the fossil spines of Echinidans), yet their organic structure is often most perfectly preserved. In the circular stems of Encrinites, the texture of

1. Lectures on Histology," vol. ii, p. 234.

the calcareous network is uniform, or nearly so, throughout; but in the pentangular Pentacrini, a certain figure or pattern is formed by variations of texture in different parts of the transverse section; and the patterns, though formed upon one general plan, are sufficiently diverse in different species, to enable these to be recognized by the examination of a transverse section of a single joint of the stem.

317. The structure of the shells, spines, and other solid parts of the skeleton of Echinodermata can only be displayed by thin sections, made upon the general plan already described (§§ 109, 110). But their peculiar texture requires that certain precautions should be taken; in the first place, in order to prevent the section from breaking, whilst being reduced to the desirable thinness; and in the second, to prevent the interspaces of the network from being clogged by the particles abraded in the reducing process. A section of the shell, spine, or other portion of the skeleton, should first be cut with a fine saw, and rubbed on a flat file until it is about as thin as an ordinary card, after which it should be smoothed on one side by friction with water on a Water-of-Ayr stone. It should then be carefully dried, first on white blotting-paper, afterwards by exposure for some time to a gentle heat, so that no water may be retained in the interstices of the network, which would oppose the complete penetration of the balsam. Next, it is to be attached to a glass slip by balsam hardened in the usual manner; but particular care should be taken, first, that the balsam be brought to exactly the right degree of hardness, and second, that there be enough, not merely to attach the specimen to the glass, but also to saturate its substance throughout. The right degree of hardness is that at which the cement can be with difficulty indented by the thumb-nail; if it be made harder than this, it is apt to chip off the glass in grinding, so that the specimen also breaks away; and if it be softer, it holds the abraded particles, so that the openings of the network becomes clogged with them. If, when rubbed down nearly to the required thinness, the section appears to be uniform and satisfactory throughout, the reduction may be completed without displacing it; but if (as often happens) some inequality in thickness should be observable, or some minute air-bubbles should present themselves between the glass and the under surface, it is desirable to loosen the specimen by the application of just enough heat to melt the balsam (special care being taken to avoid the production of fresh air-bubbles), and to turn it over so as to attach the side last polished to the glass, taking care to remove or to break with the needle-point any airbubbles that there may be in the balsam covering the part of the glass on which it is laid. The surface now brought uppermost is then to be very carefully ground down; special care being taken to keep its thickness uniform through every part (which may be even better judged of by the touch than by the eye), and to

carry the reducing process far enough, without carrying it too far. Until practice shall have enabled the operator to judge of this by passing his finger over the specimen, he must have continual recourse to the microscope during the later stages of his work; and he should bear constantly in mind, that, as the specimen will become much more transparent when mounted in balsam and covered with glass, than it is when the ground surface is exposed, he need not carry his reducing process so far as to produce at once the entire transparency he aims at, the attempt to accomplish which would involve the risk of the destruction of the specimen. In "mounting" the specimen, liquid balsam should be employed, and only a very gentle heat (not sufficient to produce air-bubbles, or to loosen the specimen from the glass) should be applied; and if, after it has been mounted, the section should be found too thick, it will be easy to remove the glass cover, and to reduce it further, care being taken to harden the balsam which has been newly laid on, to the proper degree.

318. If a number of sections are to be prepared at once (and it is often useful to do this for the sake of economy of time, or in order to compare sections taken from different parts of the same spine), this may be most readily accomplished by laying them down, when cut off by the saw, without any preliminary preparation save the blowing the calcareous dust from their sur faces, upon a thick slip of glass well covered with hardened balsam; a large proportion of its surface may thus be occupied by the sections attached to it, the chief precaution required being that all the sections come into equally close contact with it. Their surfaces may then be brought to an exact level, by rubbing them down, first upon a flat piece of grit (which is very suitable for the rough grinding of such sections), and then upon a large Water-of-Ayr stone whose surface is "true." When this level has been attained, the ground surface is to be well washed and dried, and some balsam previously hardened is to be spread over it, so as to be sucked in by the sections, a moderate heat being at the same time applied to the glass slide; and this being increased to a sufficient degree to loosen the sections without overheating the balsam, the sections are to be turned over one by one, so that the ground surfaces are now to be attached to the glass slip, special care being taken to press them all into close contact with it. They are then to be very carefully rubbed down. until they are nearly reduced to the required thinness; and it, on examining them from time to time, their thinness should be found to be uniform throughout, the reduction of the entire set may be completed at once; and when it has been carried suffi ciently far, the sections, loosened by warmth, are to be taken up upon a camel-hair brush dipped in turpentine, and transferred to separate slips of glass whereon some liquid balsam has been previously laid, in which they are to be mounted in the usual man

ner. It more frequently happens, however, that, notwithstanding every care, the sections, when ground in a number together, are not of uniform thickness, owing to some of them being underlaid by a thicker stratum of balsam than others are; and it is then necessary to transfer them to separate slips, before the reducing process is completed, attaching them with hardened balsam, and finishing each section separately.

319. It now remains for us to notice the curious and often very beautiful structures, which represent, in the order Holothurida, the solid calcareous skeleton of the orders already noticed. All the animals belonging to this order are distinguished by the flexibility and absence of firmness of their envelopes; and excepting in the case of certain species which have a set of calcareous plates, supporting teeth, disposed around the mouth, very much as in the Echinida, we do not find among them any representation that is apparent to the unassisted eye, of that skeleton which constitutes so distinctive a feature of the class generally. But a microscopic examination of their integument at once brings to view the existence of great numbers of minute isolated plates, every one of them presenting the characteristic reticulated structure, which are set with greater or less closeness in the substance of the skin. Various forms of the plates which thus present themselves in Holothuria are shown in Fig. 240; and

FIG. 240.

Calcareous plates in skin of Holothuria.

at A is seen an oblique view of the kind marked a, more highly magnified, showing the very peculiar manner wherein one part is superposed on the other, which is not at all brought into view when it is merely seen through in the ordinary manner. In the Synapta, one of the long-bodied forms of this order, which does not occur upon our own coasts, but is abundant in the Adriatic Sea, the calcareous plates of the integument have the regular form shown at A, Fig. 241; and each of these carries the curious anchor-like appendage, c, which is articulated to it by the notched piece at the foot, in the manner shown (in side view) at B. The anchor-like appendages project from the surface of the skin, and may be considered as representing the spines of Echinida. Nearly allied to the Synapta is the Chirodota, of which one species (the C. digitata), although previously accounted a very rare inhabi

tant of our seas, has lately been found in considerable numbers at Torquay (by Mr. Kingsley), and might probably be met with

Calcareous skeleton of Synapta:-A, plate imbedded in skin; B. the same, with its anchor-like spine attached; c, anchor like spine separated.

more frequently if carefully searched for. Not having had the opportunity of examining a specimen of this animal, the Author

FIG. 242.

violacea.

is unable to say whether or not its integument possesses the very remarkable wheel-like plates, represented in Fig. 242, which are found in the skin of Chirodota violacea, a species inhabiting the Mediterranean. These plates are objects of singular beauty and delicacy, being especially remarkable for the very minute notching (scarcely to be discerned in the figures without the aid of a magnifying glass) which is traceable round the inner margin of their "tires." There can be scarcely any reasonable doubt, that every member of this order has some kind of calcareous skeleton, disposed in a manner conformable to the examples now cited; and it would be very valuable to determine how far the very marked peculiarities by which they are respectively distinguished, are characteristic of genera and species. The plates may be obtained separately, by the usual method of treating the skin with a solution of potass; and they should be mounted in Canada balsam. But their position in the skin can only be ascertained by making sections of the integument, both vertical and parallel to its surface; and these sections, when dry, are most advantageously mounted in the same medium, by which their transparency is greatly increased. All the objects of this class are most beautifully displayed by the black-ground illumination (§§ 61, 62); and the same method, when applied to very thin sections of Echinus-spines, brings out some effects of marvellous beauty.

320. Echinoderm Larvae.-We have now to notice that most remarkable set of objects, furnished to the Microscopic inquirer by the larval forms of this class, for our present knowledge of which, imperfect as it still is, we are almost entirely indebted to