The most lowly animals which undoubtedly possess the power of perceiving movement are certain marine Chaetopod worms (Branchiomma). They live in tubes, constructed by themselves, from which only their heads project, which bear numerous, long tentacles. Each tentacle has at its extremity a compound eye. If one makes a rapid movement with the hand in front of the aquarium, every worm retreats into its tube with lightning rapid ity. That this is a typical case of the visual perception of move ment is proved by the fact that darkness or light, produced by the switching off or on of an electric lamp, without movement, makes no impression whatever upon the animals.
That the lower animals perceive and take heed of motionless objects is really only to be observed when they have to turn aside from their path on account of them. When an insect flies about in a forest it must not run the risk of colliding with every tree. This also applies to the deliberately moving gasteropods.
We can well observe in these slow-moving creatures how skil fully they avoid the obstacles which may be placed in their path.
This is also the case if we place between the animals and the obstacle a glass screen, which excludes all impressions of it other than visual ones. We do not know as yet how widespread this "avoiding reaction" is in the animal kingdom. On the other hand, visual perception of direction is almost universal. This power enables the animal to pursue an orderly path in a straight line. It hardly requires to be proved that it is a necessity of life for all freely-moving animals to crawl, fly or swim straight ahead. It suffices to remember that, without this power, an animal runs the risk of moving round and round in circles, in a quite aimless manner. Mephistopheles says in "Faust":— I tell you, the man who speculates, Is like a beast upon a barren heath, Led in a circle by an evil spirit, While just beyond are spreading, fresh, green fields.
We can conjure up this evil spirit in our experiments by con ducting them in darkness. if we allow small insects to creep about in the dark on a piece of glass lightly smeared with soot, we can see from their tracks that they have crawled here and there, not indeed in circles, but in a totally aimless manner, without moving far from one place (see fig. 13). We can increase this restriction of movement by am putating one of the legs of the insect ; it then frequently runs in a narrow spiral, always turn ing towards the same side. As soon as we admit light into the darkened chamber, however, matters are instantly changed; the creature runs away from the light in a course straight as a line.
Visual Sense of Direction.—These experiments prove the existence of the visual perception of direction. This applies to human beings also. It has long been known that a man in desert or snow covered regions, or in dense brush, unable to use his sight for orientation, moves round in circles, and, after walking for hours, finds himself back at the point from which he started.
The manner in which man and the higher animals orientate themselves in space by means of sight is generally known. Quite unconsciously they choose some prominent object, such as a tree, for which they aim; on approaching this they select as their goal another, more distant object, lying in the same direc tion.
The lower animals regulate their movements on a different principle. They shape their course in relation to the light.
This method of orientation is well illustrated by the following example. Xenophon frequently writes in his "Anabasis":—"We marched keeping the sun on our right." That is to say, we can in a quite unfamiliar region, travel in a straight course in a particular direction, if we take care that the rays of the sun and the line of march enclose a con stant angle. What man does in telligently the lower animals ac complish by blind instinct.
The particular motion with re lation to the light may be demon strated experimentally in two ways. A small candle is put on a table in a dark room, and the animal to be studied is placed about half a metre distant from the light. (Some small beetles or caterpillars are good subjects for this experiment.) The ani mal begins by running past the light, apparently without heeding it ; if, however, we suddenly take up the light and place it on the other side of the animal, the latter turns in an angle of 18o°, and continues its way in the opposite direction to that previously followed (fig. 14). This very pretty experiment does not succeed with all species. The following is simpler, and can be carried out with all sorts of insects, crustaceans, and gastero pods. The animal is placed on a table near the light, and left to its own devices; it may then be observed that, under the influence of the light which radiates outwards in a circle, the animal per forms a circus-movement round the source of illumination. It moves, therefore, in such a way that the angle formed by the light rays and its path remains constant. In this instance the angle is roughly a right angle; the animal is able, however, to change it at will. For example, it may crawl in such a way that the light falls upon its eyes obliquely, from behind ; it then moves in a spiral which gradually removes it from the light (see fig. 15). The long known fact that nocturnal insects are attracted by a lighted lamp is connected with this particular type of movement. Usually, on their nocturnal flights, they orientate themselves by the moon, the clouds, or other far distant sources of light. If, however, they come by chance within the influence of a near source of light, such as an electric arc-lamp, they are compelled to fly around the artificial light in circles or in spirals. Often enough they come thus into the immediate neighbourhood of the light, and are burnt.