CHAPTER X.

STAR-FISH AND SEA-URCHINS.

_Structure of Star-fish and Sea-Urchins._

We shall now proceed to consider in the organization of the Echinodermata a type of nervous system which is more highly developed than that of the Medusæ. In conducting this research, I was joined by my friend Professor J. Cossar Ewart, to whose unusual skill and untiring patience the anatomical part of the inquiry is due. But here, as formerly, I shall devote myself to the physiology of the subject, as it is not possible within the limits assigned to this volume to travel further into morphology than is necessary for the purpose of rendering the experiments intelligible. I shall therefore begin by seeking to give merely such a general idea of the structure of the Echinodermata as is necessary for this purpose.

As we all know, a Star-fish consists of a central disc and five radiating arms (Fig. 32). Upon the whole of the upper surface there occur numerous calcareous nodules embedded in the soft flesh, and supporting short spines. One of these nodules is much larger than any of the others, is constant in position, and is called the madreporic tubercle (Fig. 32, _m_). Continuing our examination of the upper surface, we may observe, when we use a lens, a number of small pincer-like organs scattered about between the calcareous nodules, or attached to the spines; these are known as the pedicellariæ. Each consists of a stalk serving to support a pair of forceps or pincers, and the whole being provided with muscles, the stalk is able to sway about and the pincers to open and shut (Fig. 33). The entire mechanism is therefore clearly adapted to seizing and holding on to something; but what it is that these curious organs are thus adapted to seize, and therefore of what use they are in the economy of the animal, has long been a standing puzzle to naturalists. I hope presently to be able to show that we have succeeded in doing something towards the solution of this puzzle.

Turning now to the under surface of our Star-fish (Fig. 34), we observe that the mouth is situated in the centre of the disc, and that from this mouth as a centre there radiate five grooves or furrows, which severally extend to the tips of each of the five rays. On each side of these grooves there are numerous actively moving membraneous tubes, which may be protruded or retracted by being filled or emptied with fluid. These are used for crawling, and I shall therefore call them the feet, or pedicels.

So much, then, for the external surface of a Star-fish. If, now, we examine the internal structure, we find that the central mouth leads by a short oesophagus into a central stomach, and that this in turn communicates with the intestine, which terminates in an orifice on the dorsal surface. Springing from the intestine at its origin, there are five tubes, each of which divides into two, and the five pairs of tubes thus formed extend into the five rays; numerous blind processes grow out from these tubes, and give rise to glandular structures, which probably perform the functions of a liver.

When a section is made across the base of one of the arms, the furrows or grooves before mentioned are seen to be formed of two rows of plates connected together so as to compose a series of structures not unlike the couples of an ordinary roof. These so-called ambulacral plates rest on horizontal spine-bearing plates, from which other larger plates extend upwards to form the sides of the arms.

In a living Star-fish the tube-feet or pedicels already mentioned are seen projecting from each side of the ambulacral groove; and, with the exception of a few at the tip of each arm, all the tube-feet terminate in a well-formed sucker, by means of which they can be firmly fixed to a flat surface (Fig. 35).

If we wish to understand the structure and mechanism of this locomotor or ambulacral system--which, I may observe in passing, is of special interest from the fact that as a mechanism it is unique in the animal kingdom--we must resort to dissection. We then find that each of the tube-feet is provided in its membraneous walls with a number of annular or ring-shaped muscular fibres; when these fibres contract, the fluid contained in the tube is forced back, while, conversely, when these fibres relax, the fluid runs into the tube. If the contraction of these fibres is strong, the tube shrinks up entirely, _i.e._ is retracted within the body of the animal; but if the contraction of the fibres is not so strong, the tube is only shortened. If, before its shortening, its terminal expansion, or sucker, has been applied to any flat surface, the effect of the shortening is to cause the sucker to adhere to the flat surface, in consequence of the pressure of the surrounding sea-water being greater than that of the fluid within the shortened tube. In this way, by alternately contracting and relaxing the muscular fibres in the walls of a tube-foot, a Star-fish is able alternately to cause the terminal sucker to fasten upon and to leave go of any flat surface upon which the animal may be crawling. In other words, when the tube-foot is about to form its attachment to a flat surface, it is fully distended with fluid; but when the terminal sucker touches the flat surface, this fluid is partly withdrawn, so causing the sucker to adhere.

When we dissect out one of these tube-feet, we find that at its base, within the body of the animal, it bifurcates into two branches. One of these branches passes immediately into a closed sac (Fig. 36, _f_), while the other passes into a large tube (Fig. 36, _k_), which runs all the way from base to tip of the ray, receiving in its course similar branches from all the tube-feet in the ray. This common or radial tube itself opens into a circular tube (Fig. 36, _e_) surrounding the mouth of the animal (Fig. 36, _m_). This circular tube therefore receives five radial tubes--one from each of the five rays--and is likewise in communication with a number of membraneous sacs (Fig. 36, _c_, _d_), resembling in their structure (though larger in size) those which occur at the base of each of the tube-feet. The function both of these sacs and of those at the base of each tube-foot is the same, namely, that of acting as reservoirs of the fluid when this is expelled from the tube-feet. Moreover, all these membraneous sacs are provided with ring-shaped muscular fibres in their membraneous walls, which therefore serve as antagonists to the ring-shaped muscles which occur in the membraneous walls of the tube-feet; that is to say, when the muscles of the reservoirs contract (Fig. 36, _c_, _d_, _f_), the pressure in the tube-feet is increased, and when these muscles relax, that pressure is diminished. The animal is thus furnished with the means of varying the head of pressure in its tube-feet, either locally or universally.

The circular tube surrounding the mouth communicates at one point with a calcareous tube (Fig. 36, _a_), which runs straight to the dorsal surface of the animal, and there terminates in the madreporic tubercle, to which I have already directed attention (Fig. 32, _m_, and Fig. 36, _m_). Thus it will be seen that all the pedicels of all the rays are in communication, by means of a closed system of tubes, with this madreporic tubercle. It has therefore been surmised that the function of this tubercle is that of acting as a filter to the sea-water which in large part constitutes the fluid that fills the ambulacral system. We have been able to prove that this surmise is correct; for we found that if we injected any part of the ambulacral system with coloured fluid--maintaining the injection for several hours at as great a pressure as the tubes would stand without rupturing--the coloured fluid found its way up the calcareous tube to the madreporic tubercle, on arriving at which it slowly oozed through the porous substance of which that tubercle consists.

Such, then, is the so-called ambulacral system of the Star-fish. Passing over another system of vessels which I need not wait to describe (Fig. 36, _g_, _h_, _l_), we come next to the nervous system. This is disposed on a very simple plan. It consists of a pentagonal ring surrounding the mouth, from which a nerve-trunk passes into each of the five rays, to run along the ambulacral groove as far as the extreme tip of the ray, where it ends in a small red pigmented spot, about which I shall have more to say presently. Each of these five radial nerves gives off in its course a number of delicate branches to the tube-feet.

_Modifications of the Star-fish Type._

So much, then, for the structure of the common Star-fish. I must next say a few words on the remarkable modifications which this structure undergoes in different members of the Star-fish group.

In some species the size of the central disc is increased so as to fill up the interspaces between the rays, the whole animal being thus converted into the form of a pentagon. In other species, again, the reverse process has taken place, the rays having become relatively longer, and being at the same time very active; they look like five little snakes joined together by a circular disc (Fig. 37). Again, in another species the rays have begun to branch, these branches again to branch, and so on till the whole animal looks like a mat. But the most extreme modifications are attained in the sea-cucumbers and lily-stars (Fig. 38). Without, however, waiting to consider these, I shall go a little more particularly into the modification of Star-fish structure which is presented by the sea-urchin, or Echinus (Fig. 39).

Externally, the animal presents the form of an orange, and is completely covered with a large number of hard calcareous spines, on which account it derives its scientific name of Echinus, or hedgehog (the spines have been removed from the larger portion of the specimen represented in Fig. 39). In the living animal these spines are fully movable in all directions, each being mounted on a ball-and-socket joint, and provided with muscles at its base. On the external surface, besides the spines, we meet with pedicellariæ (Fig. 33 magnified), and also with the madreporic tubercle (Fig. 39, _m_). The pedicellariæ in their main features resemble those which occur in the Star-fish, though considerably larger, and the ambulacral system is constructed upon the same plan. If we shave off the spines and pedicellariæ (Fig. 39), we find that we come to a hard shell, which, if we break, we find to be hollow and filled with fluid (Fig. 40). The fluid closely resembles sea-water, but is, nevertheless, richly corpusculated; it coagulates when exposed to the air, and otherwise shows that it is something more than mere sea-water. If we look closely into the shell which has been deprived of its spines, we find that it is composed of a great number of small hexagonal plates (Fig. 41), the edges of which fit so closely together that the whole shell is converted into a box, which, when the animal is alive, is water-tight, as we have proved by submitting the contained fluid to hydrostatic pressure, under which circumstances there is no leakage until the pressure is sufficient to burst the shell. Nevertheless, if we look closely at the dried shell of an Echinus, we shall see that it is not an absolutely closed box; for we shall see that the hexagonal plates are so arranged as to give rise to five double rows of holes or pores (Fig. 41), which extend symmetrically from pole to pole of the animal (Fig. 39). It is through these holes that the tube-feet are protruded; so that if we imagine a pentagonal species of Star-fish to be curved into the shape of a hollow spheroid, and then converted into a calcareous box with holes left for its feet to come through, we should have a mental picture of an Echinus. It would only be necessary to add the curious apparatus of teeth (Figs. 40 and 42), which occurs in the Echinus, to increase the size of the spines and pedicellariæ, and to make a few other such minor alterations; but in all its main features an Echinus is merely a Star-fish with its five rays calcified and soldered together so as to constitute a rigid box.

This echinoid type itself varies considerably among its numerous constituent species as to size, shape, length and thickness of the spines, etc.; but I need not wait to go into these details. Again, merely inviting momentary attention to the developmental history of these animals, I may remark that the phases of development through which an individual Echinoderm passes are not less varied and remarkable than are the permanent forms eventually assumed by the sundry species.

_Natural Movements._

Turning now to the physiology of the Star-fish group, I shall begin by describing the natural movements of the animals.

Taking the common Star-fish as our starting-point, I have already explained the mechanism of its ambulacral system. The animals usually crawl in a determinate direction, and when in the course of their advance the terminal feet of the advancing ray--which are used, not as suckers, but as feelers, protruded forwards--happen to come into contact with a solid body, the Star-fish may either continue its direction of advance unchanged, or may turn towards the body which it has touched. Thus, for instance, while crawling along the floor of a tank, if the terminal feet at the end of a ray happen to touch a perpendicular side of the tank, the animal may either at once proceed to ascend this perpendicular side, or it may continue its progress along the floor, feeling the perpendicular side with the end of its rays perhaps the whole way round the tank, and yet not choosing, as it were, to ascend. In the cases where it does ascend and reaches the surface of the water, a Star-fish very often performs a number of peculiar movements, which we may call acrobatic (Fig. 43). On reaching the surface, the animal does not wish to leave its native element--in fact, cannot do so, because its sucking feet can only act under water--and neither does it wish again to descend into the levels from which it has just ascended. It, therefore, begins to feel about for rocks or sea-weeds at the surface, by crawling along the side of the tank, and every now and then throwing back its uppermost ray or rays along the surface of the water to feel for any solid support that may be within reach. If it finds one, it may very likely attach its uppermost rays to it, and then, letting go its other attachments, swing from the one support to the other. The activity and co-ordination manifested in these acrobatic movements are surprising, and give to the animal an almost intelligent appearance.

In Astropecten the ambulacral feet have become partly rudimentary, inasmuch as they have lost their terminal suckers (Fig. 44). These Star-fish, therefore, assist themselves in locomotion by the muscular movements of their rays, while they use their suckerless feet to run along the ground somewhat after the manner of centipedes. It is to be noticed, however, that although the feet have lost their suckers, the Star-fish is still able to make them adhere to solid surfaces in a comparatively inefficient manner, by constricting the tube on one side after it has brought this side into opposition with the solid surface (Fig. 45).

In the Brittle-stars the ambulacral feet have been still more reduced to rudiments, and are of no use at all, either as suckers or for assisting in locomotion. These Star-fish have, therefore, adopted another method of locomotion, and one which is a great improvement upon the slow crawling of other members of the Star-fish group. The rays of the Brittle-stars are very long, flexible, and muscular, and by their combined action the animal is able to shuffle along flat horizontal surfaces. When it desires to move rapidly, it uses two of its opposite arms upon the horizontal floor with a motion like swimming (Fig. 46); at each stroke the animal advances with a leap or bound about the distance of two inches, and as the strokes follow one another rapidly, the Star-fish is able to travel at the rate of six feet per minute. A common Star-fish, on the other hand, with its slow crawling method of progression, can only go two inches per minute. Some of the Comatulæ, in which the muscularity of the rays has proceeded still further, are able actually to swim in the water by the co-ordinated movements of their rays.[39]

[39] In this case the locomotion of a Star-fish comes to be performed on the same plan or method as that of a Jelly-fish--the five rays performing, by their co-ordinated action, the same function as a swimming-bell. It is a curiously interesting fact, that although no two plans or mechanisms of locomotion could well be imagined as more fundamentally distinct than those which are respectively characteristic of these two groups of animals, nevertheless in this particular case and in virtue of special modification, a Star-fish should have adopted the plan or mechanism of a Jelly-fish.

The Echinus crawls in the same way as the common Star-fish; but besides its long suckers it also uses its spines, which by their co-ordinated action push the animal along. The suckers, moreover, in being protruded from all sides of a globe instead of from the under side of a flat organism, are of much more use as feelers than they are in the Star-fish. Therefore, while advancing, the feet facing the direction of advance are always kept extended to their fullest length, in order to feel for any object which the animal may possibly be approaching. When a perpendicular surface is reached, the Echinus may either ascend it or not, as in the case of the Star-fish. While walking, the animal keeps pretty persistently in one direction of advance. If it be partly rotated by the hand, it does not continue in the same direction, but continues its own movements as before; so that, for instance, if it is turned half round, it will proceed in a direction opposite to that in which it had previously been going. When at rest, some of the feet are used as anchors, and others protruded as feelers.

Regarded from the standpoint of the evolutionist, we have here an interesting series of gradations. At one end of the series we have the Echinus with its rays all united in a box-like rigid shell. At the other end of the series we have the Brittle-stars and Comatulæ with their highly muscular and mobile rays. Midway in the series we have Astropecten and the common Star-fish, where the rays are flexible and mobile, though not nearly so much so as in the Brittle-stars. Now, the point to observe is, that in correlation with this graduated difference in the mobility of the rays, there is a correspondingly graduated difference in the development of the ambulacral system of suckers. For in Echinus this system is seen in its most elaborate and efficient form; in the common Star-fish the suckers are still the most important organs of locomotion, though the muscularity of the rays has begun to tell upon the development of the specially ambulacral system, the suckers not being so long or so powerful as they are in Echinus. Lastly, the Brittle-stars and Comatulæ have altogether discarded the use of their sucking feet in favour of the much more efficient organs of locomotion supplied by their muscular rays; and, as a consequence, their feet have dwindled into useless rudiments, while the rays have become limb-like in their activity.

There is only one other point in connection with the natural movements of the Echinodermata which it is necessary for me to touch upon. All the species when turned upon their backs are able again to right themselves; but seeing, as I have just observed, that the organs of locomotion in the different species are not the same, the methods to which these species have to resort in executing the righting manoeuvre are correspondingly diverse. Thus, the Brittle-stars can easily perform the needful manoeuvre by wriggling some of their snakelike arms under the inverted disc, and heaving the whole body over by the mere muscularity of these organs. The common Star-fish, however, experiences more difficulty, and executes the manoeuvre mainly by means of its suckers. That is to say, it twists round the tip of one or more of its rays (Fig. 47) until the ambulacral feet there situated are able to get a firm hold of the floor of the tank (_a_); then, by a successive and similar action of the ambulacral feet further back in the series, the whole ray is twisted round (_b_), so that the ambulacral surface of the end is applied flat against the floor of the tank (_c_). The manoeuvre continuing, the semi-turn or spiral travels progressing all the way down the ray. Usually two or three adjacent rays perform this manoeuvre simultaneously; but if, as sometimes happens, two opposite rays should begin to do so, one of them soon ceases to continue the manoeuvre, and one or both of the rays adjacent to the other takes it up instead, so assisting and not thwarting the action. The spirals of the co-operating rays being invariably turned in the same direction (Fig. 47, _a_, _b_, and _c_), the result is, when they have proceeded sufficiently far down the rays, to drag over the remaining rays, which then abandon their hold of the bottom of the tank, so as not to offer any resistance to the lifting action of the active rays. The whole movement does not occupy more than half a minute. As a general rule, the rays are from the first co-ordinated to effect the righting movement in the direction in which it is finally to take place--the rays which are to be the active ones alone twisting over, and so twisting that all their spirals turn in the same direction.

A Star-fish (Astropecten) which is intermediate between the Brittle-star and the common Star-fish, in that its ambulacral feet are partly aborted (having lost their suckers, as shown in Fig. 44) and its rays more mobile than those of the common Star-fish, rights itself after the manner shown in Fig. 48, where the animal is represented as standing on the tips of four of its rays, while the fifth one is just about to be thrown upwards and over the others, in order to carry with it the two adjacent rays, and so eventually to overbalance the system round the fulcrum supplied by the tips of the other two rays, and thus bring the animal down upon its ventral surface.

But it is in the case of Echinus that these righting movements become most interesting, from the fact that they are so much more difficult to accomplish than they are in the case of the Star-fishes. For while a Star-fish is provided with flat, flexible, and muscular rays, comprising a small and light mass in relation to the motive power, an Echinus is a rigid, non-muscular, and globular mass, whose only motive power available for conducting the manoeuvre is that which is supplied by its relatively feeble ambulacral feet. It is, therefore, scarcely surprising that unless the specimens chosen for these observations are perfectly fresh and vigorous, they are unable to right themselves at all; they remain permanently inverted till they die. But if the specimens are fresh and vigorous, they are sooner or later sure to succeed in righting themselves, and their method of doing so is always the same. Two, or perhaps three, adjacent rows of suckers are chosen out of the five, as the rows which are to accomplish the task (Fig. 49). As many feet upon the rows as can reach the floor of the tank are protruded downwards and fastened firmly to the floor; their combined action then serves to tilt the globe slightly over in their own direction, the anchoring feet on the other or opposite rows meanwhile releasing their hold of the tank to admit of this tilting (Fig. 50). The effect of this tilting is to enable the next feet in the active ambulacral rows to touch the floor of the tank, and, when they have established their hold, they assist in increasing the tilt; then the next feet in the series lay hold, and so on, till the globe slowly but steadily rises upon its equator (Fig. 51). The difficulty of raising such a heavy mass into this position by means of the slender motive power available can be at once appreciated on witnessing the performance, so that one is surprised, notwithstanding the co-ordination displayed by all the suckers, that they are able to accomplish the work assigned to them. That the process is in truth a very laborious one is manifest, not only from the extreme slowness with which it takes place, but also because, as already observed, in the case of not perfectly strong specimens complete failure may attend the efforts to reach the position of resting on the equator--the Echinus, after rearing up a certain height, becoming exhausted and again falling back upon its ab-oral pole. Moreover, in some cases it is interesting to observe that when the equator position has been reached with difficulty, the Echinus, as it were, gives itself a breathing space before beginning the movement of descent--drawing in all its pedicels save those which hold it securely in the position to which it has attained, and remaining in a state of absolute quiescence for a prolonged time. It then suddenly begins to protrude all its feet again, and to continue its manoeuvre. At any time during such a period of rest, a stimulus of any kind will immediately determine a recommencement of the manoeuvre.

It will be perceived that as soon as the position just described has been attained, gravity, which had hitherto been acting in opposition to the righting movement, now begins to favour that movement. It might, therefore, be anticipated that the Echinus would now simply let go all its attachments and allow itself to roll over into its natural position But an Echinus will never let go its attachments without some urgent reason, seeming to be above all things afraid of being rolled about at the mercy of currents; and therefore in this case it lets itself down almost as slowly as it raised itself up. So gently, indeed, is the downward movement effected, that an observer can scarcely tell the precise moment at which the righting is concluded. Therefore, in the downward movement, the feet, which at the earlier part of the manoeuvre were employed successfully in rearing the globe upon its equator, are now employed successfully in preventing its too rapid descent (Fig. 52).

Several interesting questions arise with reference to these righting movements of Echinus. First of all we are inclined to ask what it is that determines the choice of the rows of feet which are delegated to effect the movements. As the animal has a geometrical form of perfect symmetry, we might suppose that when it is placed upon its pole, all the five rows of feet would act in antagonism to one another; for there seems nothing more to determine either the action or the inaction of one row rather than another. Indeed, if there were any moral philosophers among the Echinoderms, they might point with triumph to the fact of their being able to right themselves as an irrefutable argument in favour of the freedom of the Echinoderm will. "We are in form," they might say, "perfectly geometrical, and our feet-rows are all arranged with perfect symmetry; therefore there is no reason, apart from the sovereign freedom of our choice, why we should ever use one set of feet rather than another in executing this important movement." And indeed, I do not see how these Echinoderm philosophers could be answered by any of the human philosophers, who, with less mathematical data and with less physiological reason, employ analogous arguments to prove the freedom of the human will. Physiologists, however, would give these Echinoderm philosophers the same answer that they are in the habit of giving to the human philosophers, viz. that although the physiological conditions are very nicely balanced, they are never _so_ nicely balanced as to leave positively nothing to determine which rows of feet--that is to say, which sets of nerves--shall be used. And in this connection I may observe that on making a number of trials it becomes apparent in the case of certain individual specimens that they manifested a marked tendency to rotate always in the same direction, or to use the same set of foot-rows for the purpose of righting themselves. In these individual specimens, therefore, we must conclude that the foot-rows thus employed are selected because of some slight accidental prepotency or superiority over the others; the animal has, as it were, thus much individual _character_ as the result of a slight prepotency of some of its nerve-centres over the others.

Another question of still more interest arises out of these righting movements, namely, that as to their prompting cause. This question, however, I shall defer till later on, since it cannot be answered without the aid of experiments as distinguished from observation.

_Stimulation._

In now quitting our observations on the natural movements of the Echinodermata, and beginning an account of the various experiments which we have tried upon these animals, I shall first take the experiments in stimulation.

All the Echinodermata seek to escape from injury. Thus, for instance, if a Star-fish or an Echinus is advancing continuously in one direction, and if it be pricked or otherwise irritated on any part of an excitable surface facing the direction of advance, the animal immediately reverses that direction. There is one point of special interest concerning these movements of response to stimulation. The form of the animals and the distribution of the nervous system being, as I have before said, of geometrical regularity, it follows that by applying two stimuli simultaneously on two different aspects of the animal, the combined result of these two stimuli is that of furnishing a very pretty instance in physiology of the physical principle of the parallelogram of forces. Thus, for instance, if two stimuli of equal intensity be applied simultaneously at the opposite sides of a globular Echinus, the animal begins to walk in a direction at right angles to an imaginary line joining these two points. And, generally, wherever the two points of simultaneous stimulation may be situated, the direction of the animal's advance is the diagonal between them. As showing in more detail how very delicate is the physiological balancing of stimuli which may be produced in these organisms, and consequently the manner in which we are able to play, as it were, upon their geometrically disposed nervous systems in illustration of the mechanical principle of the composition of forces, I shall quote a series of observations.

"1. Scraped with a scalpel the equator of an Echinus at two points opposite to each other--animal crawled at right angles to the line of injury.

"2. Similarly scraped at the ab-oral pole--no effect. There was no reason why injury here should determine escape in one direction rather than in another.

"3. Scraped similarly near the oral pole, and half-way between pole and equator--little or no effect.

"4. Scraped in rapid succession five equatorial and equidistant injuries--Echinus crawled actively in one determinate direction; the equal and equidistant injuries all round the globe neutralized one another.

"5. Scraped a band of uniform width all the way round the equator--same result as in 4.

"6. Band of injury in same specimen was then widened in the side facing the direction of crawling--no effect. Still further widened--slight change of direction, and, after a time, persistent crawling away from the widest part of the injured zone. Repeated this experiment on other specimens by scraping round the whole equator, and simultaneously making one part of the zone of injury wider than the rest--same result; the animal crawled away from the _greatest amount_ of injury.

"7. Scraped on one side of the equator, and, after the animal had been crawling in a direct line from the source of irritation for a few minutes, similarly scraped equator on the opposite side--animal reversed its direction of crawling; it crawled away from the stimulus _supplied latest_.

"8. Scraped a number of places on all aspects of the animal indiscriminately--direction of advance uncertain and discontinuous, with a strong tendency to rotation upon vertical axis."

These observations show conclusively that the whole external surface, not only of the soft and fleshy Star-fish, but even of the hard and rigid Echinus, is everywhere sensitive to stimulation. Closer observation shows that this sensitiveness, besides being so general, is highly delicate. For if any part of the external surface of an Echinus is lightly touched with the point of a needle, all the feet, spines, and pedicellariæ within reach of that part, and even beyond it, immediately converge and close in upon the needle, grasp it, and hold it fast. This simultaneous movement of such a little forest of prehensile organs is a very beautiful spectacle to witness. In executing it the pedicellariæ are the most active, the spines somewhat slower, and the feet very much slower. The area affected is usually about half a square inch, although the pedicellariæ even far beyond this area may bend over towards the seat of stimulation, which, however, from their small size they are not able to reach.

And here we have proof of the function of the pedicellariæ--proof which we consider to be important, because, as I have before said, the use of these organs has so long been a puzzle to naturalists. In climbing perpendicular or inclined surfaces of rock, covered with waving sea-weeds, it must be of no small advantage to an Echinus to be provided on all sides with a multitude of forceps, all mounted on movable stalks, which instantaneously bring their grasping forceps to bear upon and to seize a passing frond. The frond being thus arrested, the spines come to the assistance of the pedicellariæ, and both together hold the Echinus to the support furnished by the sea-weed. Moreover the sea-weed is thus held steady till the ambulacral feet have time also to establish their hold upon it with their sucking discs. That the grasping and arresting of fronds of sea-weed in this way for the purposes of locomotion constitute an important function of the pedicellariæ, may at once he rendered evident experimentally by drawing a piece of sea-weed over the surface of a healthy Echinus in the water. The moment the sea-weed touches the surface of the animal, it is seen and felt to be seized by a number of these little grasping organs, and--unless torn away by a greater force than is likely to occur in currents below the surface of the sea--it is held steady till the ambulacral suckers have time to establish their attachments upon it. Thus there is no doubt that the pedicellariæ are able efficiently to perform the function which we regard as their chief function. We so regard this function, not merely because it is the one that we observe these organs chiefly to perform, but also because we find that their whole physiology is adapted to its performance. Thus their multitudinous number and ubiquitous situation all over the external surface of the animal is suggestive of their being adapted to catch something which may come upon them from any side, and which may have strings and edges so fine as to admit of being enclosed by the forceps. Again, the instantaneous activity with which they all close round and seize a moving body of a size that admits of their seizing it, is suggestive of the objects which they are adapted to seize being objects which rapidly brush over the surface of the shell, and therefore objects which, if they are to be seized at all, must be seized instantaneously. Lastly, we find, on experimenting upon pedicellariæ, whether _in situ_ or when separated from the Echinus, that the clasping action of the forceps is precisely adapted to the function which we are considering; for not only is the force exerted by the forceps during their contraction of an astonishing amount for the size of the organ (the serrated mandibles of the trident pedicellariæ holding on with a tenacity that can only have reference to some objects liable to be dragged away from their grasp), but it is very suggestive that this wonderfully tenacious hold is spontaneously relaxed after a minute or two. This is to say, the pedicellariæ tightly fix the object which they have caught for a time sufficient to enable the ambulacral suckers to establish their connections with it, and then they spontaneously leave go; their grasp is not only so exceedingly powerful while it lasts, but it is as a rule timed to suit the requirements of the pedicels.[40]

[40] A further proof that this is at least one of the functions of the pedicellariæ is furnished by a simple experiment. If an Echinus is allowed to attach its feet to a glass plate held just above its ab-oral pole, and this plate be then raised in the water so that the Echinus is freely suspended in the water by means of its feet alone, the animal feels, as it were, that its anchorage is insecure, and actively moves about its unattached feet to seek for other solid surfaces. Under such circumstances it may be observed that the pedicellariæ also become active, and especially so near the surface of attachment as if seeking for pieces of sea-weed. If a piece is presented to them, they lay hold upon it with vigour.

Of course the pedicellariæ may also have other functions to perform, and in a Star-fish Mr. Sladen has seen them engaged in cleaning the surface of the animal; but we cannot doubt that at least in Echinus their main function is that which we have stated.

Concerning the physiology of the pedicellariæ little further remains to be said. It may be stated, however, that the mandibles, which are constantly swaying about upon their contractile stalks as if in search for something to catch, will snap at an object only if it touches the inner surface of one or more of the expanded mandibles. Moreover, in the larger pedicellariæ, a certain part of the inner surface of the mandibles is much more sensitive to contact than is the rest of that surface; this part is a little pad about one-third of the way down the mandible: a delicate touch with a hair upon this part of any of the three mandibles is certain to determine an immediate closure of all the three. It is obvious that there is an advantage in the sensitive area, or zone, being placed thus low enough down in the length of the mandibles to ensure that the whole apparatus will not close upon an object till the latter is far enough within the grasp of the mechanism to give this mechanism the best possible hold. If, for instance, the tips of the mandibles were the most sensitive parts, or even if their whole inner surfaces were uniformly sensitive, the apparatus would be constantly closing upon objects when these merely brushed past their tips, and therefore closing prematurely for the purpose of grasping. But, as it is, the apparatus is admirably adapted to waiting for the best possible chance of getting a secure hold, and then snapping upon the object with all the quickness and tenacity of a spring-trap.

Another point worth mentioning is that if, after closure, any one or more of the mandibles be gently stroked on its outer surface near the base, all the mandibles are by this stimulation usually, though not invariably, induced again to expand. This is the only part of the whole organ the stimulation of which thus exerts an inhibitory influence on the contractile mechanism. If there is any functional purpose served by such relaxing influence of stimulating this particular part of the apparatus, we think it can only be as follows. When a portion of sea-weed brushes this particular part, it must be well below the tips of the mandibles, and therefore in a position where it, or some over-lying portion, may soon pass between the mandibles, if the latter are open; hence when touched in this place the mandibles, if closed, open to receive the sea-weed, should any part of it come within their cavity.

Turning next to experiments in stimulation with reference to the spines, I may observe that we have found these organs to be, physiologically considered, highly remarkable and interesting, from the fact that they display co-ordinated action in a degree which entitles them to be regarded as a vast multitude of limbs. Thus, for instance, if an Echinus be taken out of the water and placed upon a table, it is no longer able to use its feet for the purpose of locomotion, as their suckers are only adapted to be used under water. Yet the animal is able to progress slowly by means of the co-ordinated action of its spines, which are used to prop and push the globe-like shell along in some continuous direction. If, while the animal is thus slowly progressing, a lighted match be held near it, facing the direction of advance, as soon as the animal comes close enough to feel the heat, all the spines begin to make the animal move away in the opposite direction. Moreover, as showing the high degree in which the action of the spines is co-ordinated, I may mention that there is an urchin-like form of Echinoderm, which is called Spatangus, and which differs from the Echinus in having shorter feet and longer spines. When, therefore, a Spatangus is inverted, it is unable to right itself by means of its feet, as these are too short to admit of being used for this purpose; but, nevertheless, the animal is able to right itself by means of the co-ordinated action of its long spines, these being used successively and laboriously to prop and push the animal over in some one definite direction. The process takes a very long time to accomplish, and there are generally numerous failures, but the creature perseveres until it eventually succeeds.

Coming now to stimulation with reference to the feet, we find that when a drop of acid, or other severe stimulation, is applied to any part of a row of protruded pedicels, the entire row is immediately retracted, the pedicels retracting successively from the seat of irritation--so that if the latter be in the middle point of the series, two series of retractions are started, proceeding in opposite directions simultaneously; the rate at which they travel is rather slow. This process of retraction, however, although so complete within the ray irritated, does not extend to the other rays. But if the stimulus be applied to the centre of the disc, upon the oral surface of the animal, all the feet in all the rays are more or less retracted--the process of retraction radiating serially from the centre of stimulation. The influence of the stimulus, however, diminishes perceptibly with the distance from the centre. Thus, if weak acid be used as the irritant, it is only the feet near the bases of the rays that are retracted; and even if very strong acid be so used, it is only the feet as far as one-half or two-thirds of the way up the rays that are fully retracted--the remainder only having their activity impaired, while those near the tip may not be affected at all. If the drop of acid be placed on the dorsal, instead of the ventral surface of the disc, the effect on the feet is found to be just the converse; that is, the stimulus here applied greatly increases the activity of the feet. Further experiments show that this effect is produced by a stimulus applied anywhere over the dorsal aspect of the animal; so that, for instance, if a drop of acid be placed on the skin at the edge of a ray, and therefore just external to the row of ambulacral feet, the latter will be stimulated into increased activity; whereas, if the drop of acid had been placed a very small distance past the edge of the ray, so as to touch some of the feet themselves, then the whole row would have been drawn in. We have here rather an interesting case of antagonism, which is particularly well marked in Astropecten, on account of the active writhing movements which the feet exhibit when stimulated by an irritant placed on the dorsal surface of the animal. It may be added that in this antagonism the inhibitory function is the stronger; for when the feet are in active motion, owing to an irritant acting on the dorsal surface, they may be reduced to immediate quiescence--_i.e._ retracted--by placing another irritant on the ventral surface of the disc. Similarly, if retraction has been produced by placing the irritant on the ventral surface of the disc, activity cannot be again induced by placing another drop of the irritant on the dorsal surface.

Now, if we regard all these facts of stimulation taken together, it becomes evident that the external organs of an Echinoderm--feet, spines, and pedicellariæ--are all highly co-ordinated in their action; and therefore the probability arises that they are all held in communication with one another by means of an external nervous plexus. Accordingly we set to work on the external surface of the Echinus to see whether we could obtain any evidence of such a plexus microscopically. This we succeeded in doing, and afterwards found that Professor Lovèn had already briefly mentioned such a plexus as having been observed by him. The plexus consists of cells and fibres, closely distributed all over the surface of the shell, immediately under the epidermal layer of cells (Figs. 53, 54, 55), and it sends fibres all the way up the feet, spines, and pedicellariæ. As it seemed to us important to investigate the physiological properties of this plexus, Professor Ewart and I made a number of further experiments, an account of which will now lead us on to the next division of our subject, or that of section.

_Section._

1. _Star-fish._--Single rays detached from the organism crawl as fast and in as determinate a direction as do the entire animals. They also crawl up perpendicular surfaces, and sometimes away from injuries; but they do not invariably, or even generally, seek to escape from the latter, as is so certain to be the case with entire animals. Lastly, when inverted, separated rays right themselves as quickly as do the unmutilated organisms.

Dividing the nerve in any part of its length has the effect, whether or not the ray is detached from the animal, of completely destroying all physiological continuity between the pedicels on either side of the line of division. Thus, for instance, if the nerve be cut across half-way up its length, the row of pedicels is at once physiologically bisected, one-half of the row becoming as independent of the other half as it would were the whole ray divided into two parts: that is to say, the distal half of the row may crawl while the proximal half is retracted, or _vice versâ_; and if a drop of acid be placed on either half, the serial contraction of the pedicels in that half stops abruptly at the line of nerve-division. As a result of this complete physiological severance, when a detached ray so mutilated is inverted, it experiences much greater difficulty in righting itself than it does before the nerve is divided. The line of nerve injury lies flat upon the floor of the tank, while the central and distal portions of the ray, _i.e._ the portions on either side of that line, assume various movements and shapes. The central portion is particularly apt to take on the form of an arch, in which the central end of the severed ray and the line of nerve-section constitute the points of support (tetanus?) (Fig. 56), or the central end may from the first show paralysis, from which it never recovers. The distal end, on the other hand, usually continues active, twisting about in various directions, and eventually fastening its tip upon the floor of the tank to begin the spiral movement of righting itself. This movement then continues as far as the line of nerve-injury, where it invariably stops (Fig. 56). The central portion may then be dragged over into the normal position, or may remain permanently inverted, according to the strength of pull exerted by the distal portion; as a rule, it does not itself assist in the righting movement, although its feet usually continue protruded and mobile. Thus, the effect of a transverse section of the nerve in a ray is that of completely destroying physiological continuity between the pedicels on either side of the section.

The only other experiments in nerve-section to which the simple anatomy of a Star-fish exposes itself is that of dividing the nerve-ring in the disc; or, which is virtually the same thing, while leaving this intact, dividing all the nerves where they pass from it into the rays. In specimens mutilated by severing the nerves at the base of each of the five rays, or by dividing the nerve-ring between all the rays, the animal loses all power of co-ordination among its rays. When a common Star-fish is so mutilated it does not crawl in the same determinate manner as an unmutilated animal, but, if it moves at all, it moves slowly and in various directions. When inverted, the power of effecting the righting manoeuvre is seen to be gravely impaired, although eventually success is always achieved. There is a marked tendency, as compared with unmutilated specimens, to a promiscuous distribution of spirals and doublings, so that instead of a definite plan of the manoeuvre being formed from the first, as is usually the case with unmutilated specimens, such a plan is never formed at all; among the five rays there is a continual change of un-coördinated movements, so that the righting seems to be eventually effected by a mere accidental prepotency of some of the righting movements over others. Appended is a sketch of such un-coördinated movement, taken from a specimen which for more than an hour had been twisting its rays in various directions (Fig. 57). Another sketch is appended to show a form of bending which specimens mutilated as described are very apt to manifest, especially just after the operation. When placed upon their dorsal surface, they turn up all their rays with a peculiar and exactly similar curve in each, which gives to the animal a somewhat tulip-like form (Fig. 58). This form is never assumed by unmutilated specimens, and in mutilated ones, although it may last for a long time, it is never permanent. In detached rays this peculiar curve is also frequently exhibited; but if the nerve of such a ray is divided at any point in its length, the curve is restricted to the distal portion of the ray, and it stops abruptly at the line of nerve-section. When entire Star-fish are mutilated by a section of each nerve-trunk half-way up each ray, and the animal is then placed upon its back, the tetanic contraction of the muscles in the rays before mentioned as occurring under this form of section in detached rays, has the effect, when now occurring in all the rays, of elevating the disc from the floor of the tank. This opisthotonous-like spasm is not, however, permanent; and the distal ends of the rays forming adhesions to the floor of the tank, thy animal eventually rights itself, though much more slowly than unmutilated specimens. After it has righted itself, although it twists about the distal portions of the rays, it does not begin to crawl for a long time, and when it does so, it crawls in a slow and indeterminate manner. Star-fish so mutilated, however, can ascend perpendicular surfaces.

The loss of co-ordination between the rays caused by division of the nerve-ring in the disc is rendered most conspicuous in Brittle-stars, from the circumstance that in locomotion and in righting so much here depends upon co-ordinated muscular contraction of the rays. Thus, for instance, when a Brittle-star has its nerve-ring severed between each ray, an interesting series of events follows. First, there is a long period of profound shock--spontaneity, and even irritability, being almost suspended, and the rays appearing to be rigid, as if in tetanic spasm. After a time, feeble spontaneity returns--the animal, however, not moving in any determinate direction. Irritability also returns, but only for the rays immediately irritated, stimulation of one ray causing active writhing movements in that ray, but not affecting, or only feebly affecting, the other rays. The animal, therefore, is quite unable to escape from the source of irritation, the aimless movements of the rays now forming a very marked contrast to the instantaneous and vigorous leaping movements of escape which are manifested by unmutilated specimens. Moreover, unmutilated specimens will vigorously leap away, not only from stimulation of the rays, but also from that of the disc; but those with their nerve-ring cut make no attempts to escape, even from the most violent stimulation of the disc. In other words, the disc is entirely severed from all physiological connection with the rays.

If the nerve-ring be divided at two points, one on either side of a ray, that ray becomes physiologically separated from the rest of the organism. If the two nerve-divisions are so placed as to include two adjacent rays--_i.e._ if one cut is on one side of a ray and the other on the further side of an adjacent ray--then these two rays remain in physiological continuity with one another, although they suffer physiological separation from the other three. When a Brittle-star is completely divided into two portions, one portion having two arms and the other three, both portions begin actively to turn over on their backs, again upon their faces, again upon their backs, and so on alternately for an indefinite number of times. These movements arise from the rays, under the influence of stimulation caused by the section, seeking to perform their natural movements of leaping, which however end, on account of the weight of the other rays being absent, in turning themselves over. An entire Brittle-star when placed on its back after division of its nerve-ring is not able to right itself, owing to the destruction of co-ordination among its rays. Astropecten, under similar circumstances, at first bends its rays about in various ways, with a preponderant disposition to the tulip form, and keeps its ambulacral feet in active movement. But after half an hour, or an hour, the feet generally become retracted and the rays nearly motionless--the animal, like a Brittle-star, remaining permanently on its back. In this, as in other species, the effect of dividing the nerve-ring on either side of a ray is that of destroying its physiological connection with the rest of the animal, the feet in that ray, although still remaining feebly active, no longer taking part in any co-ordinated movement--that ray, therefore, being merely dragged along by the others.

Under this division it only remains further to be said, that section of the nerve-ring in the disc, or the nerve-trunks of the rays, although, as we have seen, so completely destroying physiological continuity in the rows of ambulacral feet and muscular system of the animal, does not destroy physiological continuity in the external nerve-plexus; for however much the nerve-ring and nerve-trunks may be injured, stimulation of the dorsal surface of the animal throws all the ambulacral feet and all the muscular system of the rays into active movement. This fact proves that the ambulacral feet and the muscles are all held in nervous connection with one another by the external plexus, without reference to the integrity of the main nerve-trunks.

2. _Echini._--_Section of external surface of shell._--If a cork-borer be applied to the external surface of the shell of an Echinus, and rotated there till the calcareous substance of the shell is reached, and therefore a continuous circular section of the over-lying tissues effected, it is invariably found that the spines and pedicellariæ within the circular area are physiologically separated from the contiguous spines and pedicellariæ, as regards local reflex excitability. That is to say, if any part of this circular area be stimulated, all the spines and pedicellariæ within that area immediately respond to the stimulation in the ordinary way; while none of the spines or pedicellariæ surrounding the area are affected. Similarly, if any part of the shell external to the circumscribed area be stimulated, the spines and pedicellariæ within that area are not affected. These facts prove that the function which is manifested by these appendages of localizing and gathering round a seat of stimulation, is exclusively dependent upon the external nerve-plexus. It is needless to add that in this experiment it does not signify of what size or shape or by what means the physiological island is made, so long as the destruction of the nervous plexus by a closed curve of injury is rendered complete. In order to ascertain whether, in the case of an unclosed curve of injury, any irradiation of a stimulus would take place round the ends of the curve, we made sundry kinds of section. It is, however, needless to describe these, for they all showed that, after injury of a part of the plexus, there is no irradiation of the stimulus round the ends of the injury. Thus, for instance, if a short straight line of injury be made, by drawing the point of a scalpel over the shell, say along the equator of the animal, and if a stimulus be afterwards applied on either side of that line, even quite close to one of its ends, no effect will be exerted on the spines or pedicellariæ on the other side of the line. This complete inability of a stimulus to escape round the ends of an injury, forms a marked contrast to the almost unlimited degree in which such escape takes place in the more primitive nervous plexus of the Medusæ.

Although the nervous connections on which the spines and pedicellariæ depend for their function of localizing and closing round a seat of stimulation are thus shown to be completely destroyed by injury of the external plexus, other nervous connections, upon which another function of the spines depends, are not in the smallest degree impaired by such injury. The other function to which I allude is that which brings about the general co-ordinated action of all the spines for the purposes of locomotion. That this function is not impaired by injury of the external plexus is proved by the fact that if the area within a closed line of injury on the surface of the shell be strongly irritated, all the spines over the whole surface begin to manifest their peculiar bristling movements, and by this co-ordinated action rapidly move the animal in a straight line of escape from the source of irritation; the injury to the external plexus, although completely separating the spines enclosed by it from their neighbouring spines as regards what may be called their local function of seizing the instrument of stimulation, nevertheless leaves them in undisturbed connection with all the other spines in the organism as regards what may be called their universal function of locomotion.

Evidently, therefore, this more universal function must depend upon some other set of nervous connections; and experiment shows that these are distributed over all the _internal_ surface of the shell. Our mode of experimenting was to divide the animal into two hemispheres, remove all the internal organs of both hemispheres (these operations producing no impairment of any of the functions of the pedicels, spines, or pedicellariæ), and then to paint with strong acid the inside of the shell--completely washing out the acid after about a quarter of a minute's exposure. The results of a number of experiments conducted on this method may be thus epitomized:--

The effect of painting the back or inside of the shell with strong acid (_e.g._ pure HCl) is that of at first strongly stimulating the spines into bristling movements, and soon afterwards reducing them to a state of quiescence, in which they lie more or less flat, and in a peculiarly confused manner that closely resembles the appearance of corn when "laid" by the wind. The spines have now entirely lost both their spontaneity and their power of responding to a stimulus applied on the external surface of the shell--_i.e._ their local reflex excitability, or power of closing in upon a source of irritation. These effects may be produced over the whole external surface of the shell, by painting the whole of the internal surface; but if any part of the internal surface be left unpainted, the corresponding part of the external surface remains uninjured. Conversely, if all the internal surface be left unpainted except in certain lines or patches, it will only be corresponding lines and patches on the external surface that suffer injury. It makes no difference whether these lines or patches be painted in the course of the ambulacral feet, or anywhere in the inter-ambulacral spaces.

The above remarks, which have reference to the spines, apply equally to the pedicellariæ, except that their spontaneity and reflex irritability are not destroyed, but only impaired.

Some hours after the operation it usually happens that the spontaneity and reflex irritability of the spines return, though in a feeble degree, and also those of the pedicellariæ, in a more marked degree. This applies especially to the reflex irritability of the pedicellariæ; for while their spontaneity does not return in full degree, their reflex irritability does--or almost in full degree.

These experiments, therefore, seem to point to the conclusions--1st, that the general co-ordination of the spines is dependent on the integrity of an internal nerve-plexus; 2nd, that the internal plexus is everywhere in intimate connection with the external; and 3rd, that complete destruction of the former, while profoundly influencing the functions of the latter, nevertheless does not wholly destroy them.

Professor Ewart therefore undertook carefully to examine the internal surface of the shell, to see whether any evidence of this internal nervous plexus could be found microscopically, and, after a great deal of trouble, he has succeeded in doing so. But as he has not yet published his results, I shall not forestall them further than to say that this internal plexus spreads all over the inside of the shell, and is everywhere in communication with the external plexus by means of fibres which pass between the sides of the hexagonal plates of which the shell of the animal is composed. Thus we can understand how it is that when a portion of the external plexus is isolated from the rest of that plexus as a result of the cork-borer experiment, the island still remains in communication with the nerve-centres which preside over the co-ordination of the spines, as proved by the fact of the Echinus using its spines to escape from irritation applied to the area included within the circle of injury to the external plexus produced by the cork-borer.

Now, where are these nerve-centres situated? We have just seen that we have evidence of the presence of such centres somewhere in an Echinus, seeing that all the spines exhibit such perfect co-ordination in their movements. Where, then, are these centres?

Seeing that in a Star-fish the rays are co-ordinated in their action by means of the pentagonal ring in the disc, analogy pointed to the nervous ring round the mouth of an Echinus as the part of the nervous system which most probably presides over the co-ordinated action of the spines. Accordingly, we tried the effect of removing this nervous ring, and immediately obtained conclusive proof that this was the centre of which we were in search; for as soon as the nervous ring was removed, the Echinus lost, completely and permanently, all power of co-ordination among its spines. That is to say, after this operation these organs were never again used by the animal for the purposes of locomotion, and no matter how severe an injury we applied, the Echinus, when placed on a table, did not seek to escape. But the spines were not wholly paralyzed, or motionless. On the contrary, their power of spontaneous movement continued unimpaired, as did also their power of closing round a seat of irritation on the external surface of the shell. The same remark applies to the pedicellariæ, and the explanation is simple. It is the external nervous plexus which holds all the spines and pedicellariæ in communication with one another as by a network; so that when any part of this network is irritated, all the spines and pedicellariæ in the neighbourhood move over to the seat of irritation. On the other hand, it is the internal plexus which serves to unite all the spines to the nerve-centre which surrounds the mouth, and which alone is competent to co-ordinate the action of all the spines for the purposes of locomotion.

It remains to consider whether the ambulacral feet exhibit any general co-ordinated action, and, if so, whether this likewise depends upon the same nerve-centre.

The fact already mentioned, that during progression an Echinus uses some of its feet for crawling and others for feeling its way, is enough to suggest that all the feet are co-ordinated by a nerve-centre. But in order to be quite sure about the fact of there being a general co-ordination among all the feet, we tried the following experiments.

I have already described the righting movements which are performed by an Echinus when the animal is inverted, and it will be remembered that in this animal the manoeuvre is effected by means of the feet alone. At first sight this might almost seem sufficient to prove the fact of a general co-ordination among the feet; but further reflection will show that it is not so. For the feet being all arranged in regular series, when one row begins to effect the rotation of the globe, it may very well be that its further rotation in the same direction is due only to the fact that the slight tilt produced by the pulling of the first feet in the series A, B, C gives the next feet in the series D, E, F an opportunity of reaching the floor of the tank; their adhesions being established, they would tend by their pulling to increase still further the tilt of the globe, thus giving the next feet in the series an opportunity of fastening to the floor of the tank, and so on. In order, therefore, to see whether these righting movements were due to nervous co-ordination among the feet, or merely to the accident of the serial arrangement of the feet, we tried the experiments which I shall now detail.

First of all we took an Echinus, and by means of a thread suspended it upside-down in a tank of water half-way up the side of the tank, and in such a way that only the feet on one side of the ab-oral pole were able to reach the perpendicular wall of the tank. These feet as quickly as possible established their adhesions to the perpendicular wall, and, the thread being then removed, the Echinus was left sticking to the side of the tank in an inverted position by means of the ab-oral ends of two adjacent feet-rows (Fig. 59). Under these circumstances, as we should expect from the previous experiments, the animal sets about righting itself as quickly as possible. Now, if the righting action of the feet were entirely and only of a serial character, the righting would require to be performed by rearing the animal upwards; the effect of foot after foot in the same rows being applied in succession to the side of the tank, would require to be that of rotating the globular shell against the side of the tank towards the surface of the water, and therefore against the action of gravity. This is sometimes done, which proves that the energy required to perform the feat is not more than a healthy Echinus can expend. But much more frequently the Echinus adopts another device, and the only one by which it is possible for him to attain his purpose without the labour of rotating upwards: he rotates laterally and downwards in the form of a spiral. Thus, let us call the five feet-rows, 1, 2, 3, 4, and 5 (Figs. 59, 60, 61), and suppose that 1 and 2 are in use near their ab-oral ends in holding the animal inverted against the perpendicular side of a tank. The downward spiral rotation would then be effected by gradually releasing the outer feet in row 1, and simultaneously attaching the outer feet in row 2 (_i.e._ those nearest to row 3, and furthest from row 1), as far as possible to the outer side of that row. The effect of this is to make the globe roll far enough to that side to enable the inner feet of row 3 (_i.e._ those nearest to row 2), when fully protruded, to touch the side of the tank. They establish their adhesions, and the residue of feet in row 1, now leaving go their hold, these new adhesions serve to roll the globe still further round in the same direction of lateral rotation, and so the process proceeds from row to row; but the globe does not merely roll along in a horizontal direction, or at the same level in the water, for each new row that comes into action takes care, so to speak, that the feet which it employs shall be those which are as far below the level of the feet in the row last employed as their length when fully protruded (_i.e._ their power of touching the tank) renders possible. The rotation of the globe thus becomes a double one, lateral and downwards, till the animal assumes its normal position with its oral pole against the perpendicular tank wall. So considerable is the rotation in the downward direction, that the normal position is generally attained before one complete lateral, or equatorial, rotation is completed.

The result of this experiment, therefore, implies that the righting movements are due to something more than the merely successive action of the series of feet to which the work of righting the animal may happen to be given. The same conclusion is pointed to by the results of the following experiment.

A number of vigorous Echini were thoroughly shaved with a scalpel over the whole half of one hemisphere, _i.e._ the half from the equator to the oral pole. They were then inverted on their ab-oral poles. The object of the experiment was to see what the Echini which were thus deprived of the lower half of three feet-rows would do when, in executing their righting manoeuvres, they attained to the equatorial position and then found no feet wherewith to continue the manoeuvre. The result of this experiment was first of all to show us that the Echini invariably chose the unmutilated feet-rows wherewith to right themselves. Probably this is to be explained, either by the general principle to which the escape from injury is due--viz. that injury inflicted on one side of an Echinoderm stimulates into increased activity the locomotor organs of the opposite side,--or by the consideration that destruction of the lower half of a row very probably induces some degree of shock in the remaining half, and so leaves the corresponding parts of the unmutilated rows prepotent over the mutilated one. Be this as it may, however, we found that the difficulty was easily overcome by tilting the animal over upon its mutilated feet-rows sufficiently far to prevent the unmutilated rows from reaching the floor of the tank. When held steadily in this position for a short time, the mutilated rows established their adhesions, and the Echinus was then left to itself. Under these circumstances an Echinus will always continue the manoeuvre along the mutilated feet-rows with which it was begun, till the globe reaches the position of resting upon its equator, and therefore arrives at the line where the shaved area commences. The animal then remains for hours in this position, with a gradual but continuous motion backwards, which appears to be due to the successive slipping of the spines--these organs in the righting movements being always used as props for the ambulacral feet to pull against while rearing the globe to its equatorial position, and in performing this function on a slate floor the spines are liable often to slip. The only other motion exhibited by Echini thus situated is that of a slow rolling movement, now to one side and now to another, according to the prepotency of the pull exerted by this or that row of ambulacral feet. Things continue in this way until the slow backward movement happens to bring the animal against some side of the tank, when the uninjured rows of ambulacral feet immediately adhere to the surface and rotate the animal upwards or horizontally, until it attains the normal position. But if care be taken to prevent contact with any side of the tank, the mutilated Echinus will remain propped on its equator for days; it never adopts the simple expedient of reversing the action of its mutilated feet-rows, so as to bring the globe again upon its ab-oral pole and get its unmutilated feet-rows into action.

From this we may conclude that the righting movements of the pedicels are due, not to the merely serial action of the pedicels, but to their co-ordination by a nerve-centre acting under a stimulus supplied by a sense of gravity; for if the movements of the pedicels were merely of a serial character, we should not expect that the equatorial position, having been attained under these circumstances, should be permanently maintained. We should not expect this, because after a while the pedicels, which are engaged in maintaining the globe in its equatorial position, must become exhausted and relax their hold, when those next behind in the series would lay hold of the bottom of the tank, and so on, the rotation of the globe thus proceeding in the opposite direction to that in which it had previously taken place. On the other hand, if the righting movements of the pedicels are due to co-ordination proceeding from a nerve-centre acting under a sense of gravity, we should expect the animal under the circumstances mentioned to remain permanently reared upon its equator; for this would allow that the nerve-centre was always persistently, though fruitlessly, endeavouring to co-ordinate the action of the absent feet.

Further, as proof that the ambulacral feet of Echinus are under the control of some centralizing apparatus when executing the righting manoeuvre, we may state one other fact. When the righting manoeuvre. is nearly completed by the rows engaged in executing it, the lower feet in the other rows become strongly protruded and curved downwards, in anticipation of shortly coming into contact with the floor of the tank when the righting manoeuvre shall have been completed (see Fig. 52, p. 280). This fact tends to show that all the ambulacral feet of the animal are, like all the spines, held in mutual communication with one another by some centralizing mechanism.

But the best proof of all that the feet in executing the righting manoeuvre are under the influence of a co-ordinating centre, is one that arose from an experiment suggested to me by Mr. Francis Darwin, and which I shall now describe. Mr. Darwin having kindly sent the apparatus which his father and himself had used in their experiments on the geotropism of plants, it was employed thus. A healthy Echinus was placed in a large bottle filled to the brim with sea-water, and having been inverted on the bottom of the bottle, it was allowed in that position to establish its adhesions. The bottle was then corked and mounted on an upright wheel of the apparatus whereby, by means of clockwork, it could be kept in continual slow rotation in a vertical plane. The object of this was to ascertain whether the continuous rotation in a vertical plane would prevent the animal from righting itself (because confusing the nerve-centres which, under ordinary circumstances, could feel by their sense of gravity which was up and which was down), or would still allow the animal to right itself (because not interfering with the serial action of the feet). Well, it was found that this rotation of the whole animal in a vertical plane entirely prevented the righting movements during any length of time that it might be continued, and that these movements were immediately resumed as soon as the rotation was allowed to cease. This, moreover, was the case, no matter what phase of the righting manoeuvre the Echinus might have reached at the moment when the rotation began. Thus, for instance, if the globe were allowed to have reached the position of resting on its equator before the rotation was commenced, the Echinus would remain motionless, holding on with its equatorial feet, so long as the rotation was kept up.

Therefore, there can be no question that the ambulacral feet are all under the influence of a co-ordinating nerve-centre, quite as much as are the spines. But, on the other hand, experiments show that the centre in this case is not of so localized a character as it is in the case of the spines; for when the nerve-ring is cut out, the co-ordination of the feet, although impaired, is not wholly destroyed. Take, for instance, the case of the righting manoeuvre. The effect of cutting out the nerve-ring is that of entirely destroying the ability to perform this manoeuvre in the case of the majority of specimens; nevertheless about one in ten continue able to perform it. Again, if an Echinus is divided into two hemispheres by an incision carried from pole to pole through any meridian, the two hemispheres will live for days, crawling about in the same manner as entire animals; if their ocular plates are not injured, they seek the light, and when inverted they right themselves. The same observations apply to smaller segments, and even to single detached rows of ambulacral feet. The latter are, of course, analogous to the single detached rays of a Star-fish, so far as the system of ambulacral feet is concerned; but, looking to the more complicated apparatus of locomotion (spines and pedicellariæ), as well as to the rigid consistence and awkward shape of the segment--standing erect, instead of lying flat--the appearance presented by such a segment in locomotion is much more curious, if not surprising, than that presented by the analogous part of a Star-fish under similar circumstances. It is still more surprising that such a fifth-part segment of an Echinus will, when propped up on its ab-oral pole (Fig. 62), right itself (Fig. 63) after the manner of larger segments or entire animals. They, however, experience more difficulty in doing so, and very often, or indeed generally, fail to complete the manoeuvre.

On the whole, then, we may conclude that the nervous system of an Echinus consists (1) of an external plexus which serves to unite all the feet, spines, and pedicellariæ together, so that they all approximate a point of irritation situated anywhere in that plexus; (2) of an internal nervous plexus which is everywhere in communication through the thickness of the shell with the external, and the function of which is that of bringing the feet, spines, and probably also the pedicellariæ into relation with the great co-ordinating nerve-centre situated round the mouth; (3) of central nervous matter which is mainly gathered round the mouth, and there presides exclusively over the co-ordinated action of the spines, and in large part also over the co-ordinated action of the feet, but which is further in part distributed along the courses of the main nerve-trunks, and so secures co-ordination of feet even in separated segments of the animal.

_Special Senses._

Before concluding, I must say a few words on the experiments whereby we sought to test for the presence in Echinoderms of the special senses of sight and smell.

We have found unequivocal evidence of the Star-fish (with the exception of the Brittle-stars) and the Echini manifesting a strong disposition to crawl towards, and remain in, the light. Thus, if a large tank be completely darkened, except at one end where a narrow slit of light is admitted, and if a number of Star-fish and Echini be scattered over the floor of the tank, in a few hours the whole number, with the exception of perhaps a few per cent., will be found congregated in the narrow slit of light. The source we used was diffused daylight, which was admitted through two sheets of glass, so that the thermal rays might be considered practically excluded. The _intensity_ of the light which the Echinoderms are able to perceive may be very feeble indeed; for in our first experiments we boarded up the face of the tank with ordinary pinewood, in order to exclude the light over all parts of the tank except at one narrow slit between two of the boards. On taking down the boards we found, indeed, the majority of the specimens in or near the slit of light; but we also found a number of other specimens gathering all the way along the glass face of the tank that was immediately behind the pine-boards. On repeating the experiment with blackened boards, this was never found to be the case; so there can be no doubt that in the first experiments the animals were attracted by the faint glimmer of the white boards, as illuminated by the very small amount of light scattered from the narrow slit through a tank, all the other sides of which were black slate. Indeed, towards the end of the tank, where some of the specimens were found, so feeble must have been the intensity of this glimmer, that we doubt whether even human eyes could have discerned it very distinctly. Owing to the prisms at our command not having sufficient dispersive power for the experiments, and not wishing to rely on the uncertain method of employing coloured glass, we were unable to ascertain how the Echinoderms might be affected by different rays.

On removing with a pointed scalpel the eye-spots from a number of Star-fish and Echini, without otherwise injuring the animals, the latter no longer crawled towards the light, even though this were admitted to the tank in abundance; but they crawled promiscuously in all directions. On the other hand, if only one out of the five eye-spots were left intact, the animals crawled towards the light as before. It may be added that single detached rays of Star-fish and fifth-part segments of Echini crawl towards the light in the same manner as entire animals, provided, of course, that the eye-spot is not injured.

The presence of a sense of smell in Star-fish was proved by keeping some of these animals for several days in a tank without food, and then presenting them with small pieces of shell-fish. The Star-fish immediately perceived the proximity of food, as shown by their immediately crawling towards it. Moreover, if a small piece of the food were held in a pair of forceps and gently withdrawn as the Star-fish approached it, the animal could be led about the floor of the tank in any direction, just as a hungry dog could be led about by continually withdrawing from his nose a piece of meat as he continually follows it up. This experiment, however, was only successful with Star-fish which had been kept fasting for several days; freshly caught Star-fish were not nearly so keen in their manifestations, and indeed in many cases did not notice the food at all.

Desiring to ascertain whether the sense of smell were localized in any particular organs, as we had found to be the case with the sense of sight, I first tried the effect of removing the five ocelli. This produced no difference in the result of the above experiment with hungry Star-fish, and therefore I next tried the effect of cutting off the tips of the rays. The Star-fish behaving as before, I then progressively truncated the rays, and thus eventually found that the olfactory sense was equally distributed throughout their length. The question, however, still remained whether it was equally distributed over both the upper and the lower surfaces. I therefore tried the effect of varnishing the upper surface. The Star-fish continued to find its food as before, which showed that the sense of smell was distributed along the lower surface. I could not try the converse experiment of varnishing this surface, because I should thereby have hindered the action of the ambulacral feet. But by another method I was able nearly as well to show that the upper surface does not participate in smelling. This method consisted in placing a piece of shell-fish upon the upper surface and allowing it to rest there. When this was done, the Star-fish made no attempt to remove the morsel of food by brushing it off with the tips of its rays, as is the habit of the animal when any irritating substance is applied to this surface. Therefore I conclude that the upper or dorsal surface of a Star-fish takes no part in ministering to the sense of smell, which by the experiment of varnishing this surface, and also by that of progressively truncating the rays, is proved to be distributed over the whole of the ventral or lower surface of the animal. For I must add that severed rays behave in all these respects like the entire organisms, although they are disconnected from the mouth and disc.

As this chapter has already extended to so great a length, I omit from it any account of some further experiments which I tried concerning the effects of nerve-poisons upon the Echinodermata. A full record of these experiments may be found in the publications of the Linnean Society.

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