dissolved, and the body drawn out at the poles. These are "sea-cucumbers." These have a vermiform body, a flexible integument with calcareous particles, an anus which is terminal in position, and an ambulacral region greatly in excess of the antambulacral region. The primitive larva, however, is, in this group, not in the form of a pluteus, but vermiform. Moreover, sometimes there are but three rows of ambulacra instead of five, the lantern is represented only by rudimentary alveoli and rotulæ, there are no spines and no pedicellariæ, and the generative organs are unsymmetrical, consisting of a bundle of blindlyending tubes, opening on one side of the neck by a solitary aperture. This order contains the only monoecious form of the class-namely, the genus Synapta.

The third order (fig. 7) is named Asteridea, and contains the star-fishes. Here the body is mostly stellate, though sometimes it is discoidal. The so-called arms are really parts of the body, into which extend sacculated processes of the alimentary canal. The ambulacra lie in deep grooves on the under surface of the so-called arms, and only on their under surface; and thus the ambulacral region is only coextensive with the antambulacral region, instead of being in excess of it. The integument is strengthened by thick and strong calcareous opicles. The primitive larva is vermiform, and the so-called arms are sometimes more than five in number. In many there are pedicellariæ, but these have only two jaws, There is no dental apparatus in the mouth. The madreporic tubercle is interradial and conspicuous, and its canal, sometimes hardened and jointed, is termed the "sand canal." The nervous system is essentially like that of the echinus, and the diverging nerves are placed superficially to the ambulacral canals, and terminate each in an eye surrounded by movable spines.

The fourth existing order consists of the Ophiuridea, or "sand stars" (fig. 8). These animals have real arms, distinct from the body (which is termed the calix), sometimes branching and sometimes provided with lateral processes. The ambulacral region is coextensive with the antambulacral region. The integument is calcareous, but not provided with pedicellariæ. There is no anus. In one important point these sand stars resemble the echini, namely, in having the primary larva in the form of a pluteus. The ambulacral vessels run along just beneath the ventral surface of the arms, but here (unlike the star-fishes) they are sheltered within the skeleton, not merely placed in grooves.

The last and most aberrant order (fig. 9) of existing echinoderms consists of the Crinoidea, all of which pass at least the early stages of their existence rooted on a stalk. Such forms abounded in that vast period during which primary and secondary rocks

were deposited, and their remains are known as stone lilies." Now, however, they form an insignificant fraction of the existing representatives of the class, and the permanently stalked form (pentacrinus) is very rare, and confined to hot climes alone. These crinoids have the arms stellate, mostly branching. The ambulacral region is coextensive with the antambulacral region, the calix is distinct from the arms, the integument is calcareous, the generative organs are numerous and external, and the primary larva is vermiform. The position of the body is the reverse of that in the echinus and star-fish, the mouth being upwards. Peculiar processes, termed cirri, project round the base of the calix in the free form comatula, and from the stalk, at intervals in the fixed and permanently stalked form pentacrinus.

It is difficult to obtain good specimens of the Holothuridea, because these creatures have the singular habit, when alarmed, of "starting" so violently as to eject the whole of their viscera. Certain of the branching forms are also exceedingly difficult to procure in a perfect state, from the extreme facility and readiness with which they spontaneously break themselves up when captured. The late Professor Edward Forbes gives a very amusing account of his unsuccessful attempt to secure a "brittle-star which he had caught in his dredge, and for which he had prepared a bucket of fresh water to kill it instantly, and so, he hoped, avoid its demolition. He says: "As I expected, a Luidia came up a most gorgeous specimen. As it does not generally break up before it is raised above the surface of the sea, cautiously and anxiously I sank my bucket to a level with the dredge's mouth, and proceeded in the most gentle manner to introduce Luidia to the purer element. Whether this cold element was too much for him, or the sight of the bucket too terrific, I know not, but in a moment he proceeded to dissolve his corporation, and at every mesh of the dredge his fragments were seen escaping. In despair I grasped the largest and brought up the extremity of an arm, with its terminating eye, the spinous eyelid of which opened and closed with something exceedingly like a wink of derision."

DESCRIPTION OF PLATE LXV.*

FIG. 1. An echinus with the spines removed from one half of the shell so as to show-a, the ambulacral plates with their pores; i, the interambulacral plates; b, tubercles for the attachment of spines.

These figures are taken by permission from specimens which form part of Professor Flower's educational series in the Museum of the Royal College of Surgeons.

FIG. 2.

FIG. 3.

FIG. 4.

FIG. 5.

FIG. 6.

FIG. 7.

FIG. 8.
FIG. 9.

Inside of the shell of an echinus to show-L, Aristotle's lantern;
an, an auricula; a, ambulacra; i, interambulacra.
Oral pole of an echinus, showing the apices of the five teeth pro-
truding from the mouth.

Apical pole of an echinus—a, ambulacra; i, interambulacra; g, one
of the five genital plates; m, madreporic tubercle, and modified
genital plate; o, ocular plates.

Parts which compose Aristotle's lantern; al, alveoli enclosing t, teeth; r, rotula; d, radii.

Cucumaria, one of the Holothuridea.

Uraster, one of the Asteridea, showing the oral surface and the ambulacra in grooves in the so-called arms.

One of the Ophiuridea: the oral surface.

Comatula, one of the Crinoidea: side view. The mouth turned downwards; c, cirri surrounding the base of the calix.

THE SUN'S CORONA.

BY RICHARD A. PROCTOR, B.A., F.R.A.S. AUTHOR OF "OTHER WORLDS THAN OURS," "HALF-HOURS WITH THE TELESCOPE," &c. &c.

D

URING the approaching total solar eclipse the sun's corona will undoubtedly be the chief object of the attention of observers. Hitherto, during great eclipses, the corona has been an object of but secondary-scarcely even of secondary-interest. Even during the American eclipse of last year, the prominences were scrutinised far more carefully than the corona. But next December all this will be changed. For the first time in the history of astronomy, the chief object of the observers of a total eclipse will be the determination of the true nature of this striking appendage; and undoubtedly such observers as journey to Spain or Sicily to view the eclipse will regard their work as a failure, unless it enables them to solve some at least of the problems presented by the solar corona.

On this account I think a brief consideration of the nature of those problems cannot fail to be of interest, while I am not without hope that the study of some of the facts which I am about to adduce may suggest modes of observation or research which may be applied successfully next December by those who are fortunate enough to view the coming eclipse.

It may seem, perhaps, to some that the very circumstance that a great eclipse is approaching, during which many skilful observers will study the phenomena of the corona, should serve to check all attempts at theorising. But as a matter of fact, the whole history of scientific progress shows how important it is that observation should not only be accompanied and followed, but in part preceded, by a process of inductive reasoning. More especially is this the case where the observation is to be made in the course of a few brief moments. Hurried as an observer of a total eclipse must necessarily feel, and startled, too, by the grandeur and solemnity of the phenomena taking place before his eyes,

it is most unfit that the consideration of what is worthiest of observation should be left to the very moment at which observation is to begin. It cannot be but that a careful consideration beforehand of the probable nature of the phenomena he is to observe, of the circumstances which, if carefully noted, may resolve doubts, and of the special parts of the heavens to which his attention should be directed, will tend materially to increase the value of his observations.

Indeed, it is only necessary to consider the records of former eclipses to see that this must be so. Without in any way slighting the observing powers of those who have handed down to us the records of great eclipses, it is impossible not to feel, as we consider what they actually accomplished, that they might readily have accomplished much more. We find the attention of each observer distracted between a variety of objects; facts are recorded which do not tend, and could not possibly tend, to elucidate any of the questions of interest which have been at issue; and, in fine, one record after another displays evidence of an eager anxiety to ascertain new truths, marred by a very imperfect recognition of the way in which that purpose could best be accomplished.

In the first place, it will be well to enquire what lessons may be educed from observations already made upon the corona, and further, what light other observations or researches may throw upon our subject.

There are three theories of the corona which have at various times been upheld by astronomers, and between which it will be well that we should endeavour to make a selection.

These theories assign to the corona very different positions in space. One places the corona around the sun, the second around our moon, the third in our own atmosphere. According to the theory (of these three) which may be finally established, we shall have three very various degrees of magnitude and importance to assign to the corona. If it is a solar appendage, its extent exceeds that of any body within the solar system, save perhaps one or two of the most remarkable comets. If it is a lunar appendage, it sinks into relative insignificance, but still has an absolute volume far exceeding that of our own earth. If, lastly, it is brought within the confines of our own atmosphere, it is not merely reduced to proportions altogether insignificant, as well absolutely as relatively, but it no longer has any real existence as a substance of any sort, any more than the beam of light which shines through clouds can be regarded as an actually existent measurable mass.

I shall take the second of these theories first in order, because it is the one we can most readily dispose of.

To Halley it seemed an acceptable theory, that the corona is