In Arbacia this form of reaction cannot be produced; the spines respond to stimuli of all degrees of intensity by convergence towards the point of stimulation.
When a general skin-irritant like dilute acetic acid, or even strong light, is applied to the skin of a Sea-urchin the spines bend alternately to all points of the compass, or, in a word, rotate. This is due to the fact that the weight of the inclined spine stretches the muscles of one side and so renders them more open to the general stimulus; these muscles in consequence, contract, and so move the spine to a new position in which other muscles are stretched, and a similar result follows. A continuation of this process brings about rotation.
When a piece of glass rod or other light object is laid on the spines of a Sea-urchin, it naturally, by its weight, presses asunder the spines and stretches their muscles on one side, thus lowering the tone. If now the skin be stimulated at any point the piece of rod will be rolled by the spines towards the point of stimulation. This is caused by the fact that the muscles of the spines holding the rod are made more receptive by being stretched, and therefore they contract more than do the others in response to the stimulation, and so the rod is rolled onwards on to the next spines, which then act in the same manner. This passage of stimulus is entirely independent of direct nervous connexion between the bases of the spines, for it will traverse at right angles a crack going clean through the shell; it is merely the result of the mechanical weight of the object and of the juxtaposition of the spines.
If the stimulation be too violent the first spines affected diverge wildly and strike their neighbours with vehemence, so arousing into activity the block musculature of these. This causes them to stand rigidly up, and so the path of the stimulus is barred.
Now the escape movements of the animal under strong stimulation which Romanes[[480]] alludes to are just an example of this handing on of stimulation from spine to spine, not by nervous connexion but by mechanical touch only; the object in this case is the substratum on which the animal lies, which is, so to speak, rolled towards the point of stimulation, or putting it otherwise, the animal is rolled away from it. Righting when upset is another example of the same phenomenon; the aboral spines are stretched by the weight of the animal, and the animal acts as if it were stimulated in the region of the periproct. When a Sea-urchin is in its normal position and is stimulated in the periproct (as for instance by a strong light), it would, according to this rule, tend to move downwards, which is of course impossible; but as the stimulus never affects all sides quite alike the result is that the Urchin rotates, turning itself ever away from the point of strongest stimulation. In the case of Strongylocentrotus lividus when living on limestone, as on the west coast of Ireland, this results in the animal excavating for itself holes in the rock, where it is safe from the action of the breakers.[[481]]
But it may be objected that no account is taken in the above description of the action of the "central nervous system," i.e. of the ring and the radial cords, and yet Romanes found that when they were removed the escape movements could not be carried out. The answer is that the central nervous system is a store-house of tone, not, as in higher animals, a controlling centre for co-ordinating the movements of the spines. When it is removed at first the escape movements can be carried out, but in a day or two all tone in the spine-muscles is lost, and then, since the tone of all is equally low, there is no tendency in those that are stretched to be more responsive than others, and hence the escape movements cannot be carried out. Sea-urchins kept in the tanks of an aquarium are apt to lose the tone of their spines owing to the poisoning of the nervous system.
The central nervous system is, however, the system which controls the movements of the tube-feet. As we have seen, extensions of the radial nerves run to the tip of each podium. Tube-feet are chiefly used in ordinary progression; when this is quickened the spines come into play exclusively. The extent to which these two organs of locomotion are used varies from genus to genus. Thus Centrostephanus uses its spines a good deal, Echinus and Strongylocentrotus very little. The last-named genus sometimes walks on its tube-feet entirely without touching the ground with its spines.
The faculty of vision in its simplest form may be defined as sensitiveness to light and shade. Now strong light acts on all Sea-urchins as a general skin irritant. They fly from it towards the darkest corner, and then if it continues the spines rotate. A number of little violet spines on the aboral pole of Centrostephanus longispinosus are especially sensitive to light, and hence are almost constantly in rotation. This is due, according to Uexküll,[[482]] to a pigment of a purple colour, which can be extracted by means of alcohol and which is decomposed by light, the products of decomposition being supposed to irritate the nerves. Centrostephanus when exposed to light becomes darker in colour. This is due to the migration outwards of amoebocytes, which carry a pigment which acts as a screen in order to prevent the valuable visual purple being too rapidly decomposed. Not all Sea-urchins, in fact very few of those living in northern waters, give a reaction to shadow. C. longispinosus is one of the few; it reacts to a shadow by converging its spines towards it. A much larger number of species inhabiting tropical waters show this reaction. It is entirely stopped if the radial nerve-cords be removed, whereas the reaction to strong light continues. The reaction to shade is strongest after a long previous exposure to light, hence Uexküll has given the following explanation of it. The continued irritation due to light, having spread to all the spines, eventually reaches the radial cords and is there stored in the bipolar nerve-cells as tone. When the light-stimulus is interrupted some of the stored tone spreads upwards to the spines, causing the weak form of spine reaction, and the spines converge.
Fig. 233.—To show character and distribution of the sphaeridia in Strongylocentrotus droëbachiensis. A, a portion of a radius, with sphaeridia, and the adjoining edge of the peristome. p, Pair of pores for a tube-foot; per, peristome; t, primary tubercle. B, an isolated sphaeridium. (After Lovén.)