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.

[Illustration: Fig. 32. Upper surface of a Star-fish

(_Astropecten_). (From Cassell's "Nat. Hist.")]

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.

[Illustration: Fig. 33.--Pedicellariæ (magnified). (From

Cassell's "Nat. Hist.")]

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.

[Illustration: Fig. 34.--Lower surface of common Star-fish.]

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.

[Illustration: Fig. 35.--The terminal portion of a tube-foot

(magnified).]

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.

[Illustration: Fig. 36.--Diagram of ambulacral system of a

Star-fish: _a_, madreporic canal; _b_, inner end; _g_, outer

end of sinus leading to circular neural vessel; _h_, from which

radial neural vessels, _l_, arise; _c d_, Polian vesicles; _f_,

ampullæ; _m_, oral aperture; _n_, madreporic plate.]

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).

[Illustration: Fig. 37.--A Brittle-star. (From Cassell's "Nat.

Hist.")]

[Illustration: Fig. 38.--A Lily-star. (From Cassell's "Nat.

Hist.")]

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.

[Illustration: Fig. 39.--An Echinus, partly denuded of its

spines. (From Cassell's "Nat. Hist.")]

[Illustration: Fig. 40.--Showing interior of Echinus shell. (From

Cassell's "Nat. Hist.")]

[Illustration: Fig. 41.--A portion of the external shell of an

Echinus denuded of spines and slightly magnified, showing the

arrangement of the plates, the balls in the ball-and-socket

joints of the spines, and the holes through which the ambulacral

feet are protruded. (From Cassell's "Nat. Hist.")]

[Illustration: Fig. 42.--Teeth of Echinus (from Cassell's "Nat.

Hist.")]

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.

[Illustration: Fig. 43.--Natural movements of a Star-fish on

reaching the surface of water.]

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).

[Illustration: Fig. 44.--A pedicel of Astropecten (magnified),

showing the absence of any terminal sucker.]

[Illustration: Fig. 45.--The same, showing the method of

extemporizing a sucker.]

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.

[Illustration: Fig. 46.--Natural movements of a Brittle-star when

proceeding along a solid horizontal surface.]

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.

[Illustration: Fig. 47.--Natural righting movements of common

Star-fish.]

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.

[Illustration: Fig. 48.--Righting movements of Astropecten.]

[Illustration: Fig. 49.]

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.

[Illustration: Fig. 50.]

[Illustration: Fig. 51.]

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).

[Illustration: 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.

[Illustration: Fig. 53. External nerve-plexus of Echinus.]

[Illustration: Fig. 54. Structure of a nerve-trunk of Echinus.]

[Illustration: Fig. 55. Nerve-cells lying among the muscular

fibres at the base of a spine in Echinus.]

_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.

[Illustration: Fig. 56. Movements performed by a detached ray of

a Star-fish, in which the central nerve-trunk is divided.]

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.

[Illustration: Fig. 57. Un-coördinated movements of a Star-fish,

in which the nerves of all the rays have been divided.]

[Illustration: Fig. 58. Form frequently assumed by Star-fish

under similar circumstances.]

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.

[Illustration: Fig. 59.]

[Illustration: Fig. 60.]

[Illustration: Fig. 61

Figs. 50, 60, and 61 are righting movements of Echinus on a

perpendicular surface.]

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.

[Illustration: Figs. 62 and 63.--Righting and ambulacral

movements of severed segments of Echinus.]

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.

FINIS.

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Transcriber's note:

The Advertisement that was originally at the front of the book

has been moved to the end. Minor spelling inconsistencies, mainly

hyphenated words, have been harmonized. Obvious typos have been

corrected. Paragraph breaks have been inserted both before and

after the table on page 148 (Chapter VII).