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."