TOUCH, SENSE OF. In the environment of every animal, the solid, impenetrable objects with which it may collide play an important part. Sensitiveness to touch, comparable to our own, is, therefore, generally developed. The manner in which an animal reacts to contact stimuli depends, principally, on the force of the contact. All such stimuli which exceed a certain degree act as repellants, and feeble, defence less animals seek to escape from them. This is evident already in the simplest animals, such as the amoeba. It is sufficient to touch one of the protruded pseudopodia of an amoeba with a pointed, glass needle to bring about its immediate retraction. Already in many infusorians there are extraordinarily de veloped tactile cilia; if another animal comes into contact with them, it causes the infusorian to swim hastily away (fig. i).
In multicellular animals the reaction to mechanical impulses differs very much, in accordance with the defensive powers of , the individual. This is sufficiently well known in the higher animals. Defenceless animals, such as earthworms, or gastero pods, flee, or withdraw into their protective shells ; on the other hand, those capable of defending themselves, such as the large Crustacea, cuttlefishes or sea-urchins meet the assault by active defensive movements. The example last mentioned is, perhaps, of sufficient interest for it to be discussed more particularly.
If we touch a sea-urchin on any part of its surface, one of three things happens. In the first case, all the spines which are situated in the neighbourhood of the part stimulated bend towards it. If some animal were the cause of the contact it would, thus, be grievously stung by the sharp spines, and put to flight. In the second case the pedicellariae come into action (fig. 2). These are pincers consisting of three small snapping blades on a pliant stalk. They turn towards the place whence the contact stimuli proceed, and wave their gaping jaws rapidly to and fro. They bite firmly into the skin of the enemy, or into its hairs, and assist, either in putting it to flight, or in holding it fast and bringing it into contact with the tube-feet round the mouth, where it is consumed. Lastly, in a third case, the whole sea urchin moves in a direction calculated, if possible, to take it out of the sphere of the stimulus.
The common crab, particularly the male, reacts to strong con tact stimuli by assuming its very characteristic defensive attitude. It turns towards its opponent and presents its widely-opened pincers. If the enemy still continues to approach, it strikes at it with great force.
Some animals are able to assume an appearance of death when attacked. The most pregnant examples of this are furnished by insects. The death-feigning reflex usually consists in the animal drawing in its legs, and remaining perfectly motionless, con tinuing thus even if maltreated in the most violent manner. In such a case, the assailant, a bird for example, thinks it is dealing with a dead thing instead of a food animal, and gives up the attack. Insects such as these, which feign death, usually resemble very closely some inanimate object, such as a piece of wood or a twig. The stick-insects provide the best known examples of this.
As was mentioned at the beginning, the sense of touch often serves to prevent an animal dashing itself against hard objects when moving from place to place. The whirligig beetle, Gyrinus, which we see skimming rapidly to and fro over the surface of the water in summer, bears eloquent witness to the degree of sensitiveness attained. The careful observer will soon perceive that these lively little creatures never collide with any object which may be floating on the surface of the water; they also do not run against one another, although, often, many may be skim ming about in a restricted space. The sense organs which render this possible are situated in the feelers, as Eggers has proved. The antennary flagella are borne free in the air; their amputation does not cause alterations in the animal's conduct. On the other hand, the second basal joint rests on the water like a float (fig. 3). It contains numerous sensory cells, bearing stylets, which are grouped together in "Johnston's organ." With the assistance of these the animal perceives in some way the proximity of solid objects ; it may, perhaps, react to the waves of water reflected to it by these objects.