There is not very much to be said on the subject of the anatomy of the sensory cells which serve for the transmission of mechanical stimuli. It has been mentioned already that, even among unicellular animals (in fusoria) tactile cilia are oc casionally to be found. (See fig. T.) In multicellular ani mals there are frequently found, projecting from the surface of the body, groups of cells bearing small hairs like cilia, which are usually considered to be tactile organs (fig. 7 and 8). Several of these organs from the skin of different kinds of Echinoderms are shown in fig. 7. In fig. 8 is represented a "sen silla" from the skin of the leech. In yet other cases we find what are called free nerve-endings (fig. 9), the nerve cells, which are deeply situated beneath the skin, sending one or many fine processes up to the outer surface. Unfortunately, in no case can it be proved definitely that the groups of sensory cells men tioned really serve for the perception of mechanical stimuli. The only way in which this might be determined, namely by remov ing the organs by an operation and experimenting with the animals thus treated, is not practicable, owing to the large number of the organs, which usually are distributed over the whole surface of the skin. This difficulty of proof occurs also when dealing with arthropods, in which the tactile organs exhibit an astonishing (fig. io B), sensory pits ( io C), cup-shaped sense organs (fig. 10 D), and so forth. In insects only there is found also a par ticular kind of sense organ, the chordotonal organs. In form they are like a cord stretched between two flat surfaces of the body wall, and, doubtless, function when these surfaces are displaced in any way. The following parts may be distinguished in them : the sensory cell with the proximal enveloping cell, the distal enveloping cell (cap cell) which connects the peculiar stylet (sco lopala) with the hypodermic, and, lastly, the ligament which spans the whole apparatus. Such organs are found singly or in groups on the body and extremities of insects. The organs which most resemble them are "Johnston's organs" in the feelers. (See fig. 4.) In the auditory organs of insects the chordotonal organs cer tainly receive the sound waves ; what purpose they serve in Johnston's organ, or in the simpler organs, is not yet known.
Since heat, as is well known, consists in a mechanical vibration of molecules, the sense of tem perature, also, must be included in the mechanical sense. It goes without saying that the higher animals possess a sense of tempera ture which is very similar to that of human beings. It is of greater interest to prove its presence in the lower animals. The simplest method of deciding whether an animal reacts to heat stimuli is by the use of the temperature organ. This consists of a narrow box, about a meter in length, the bottom of which is heated at one end, and, at the other, is cooled with ice. If a number of small animals, such as ants, are placed in the box, they avoid both the hot and the cold ends, and seek for a part in which a temperature prevails which suits them best.
Bees furnish a very fine example of an extraordinarily developed temperature sense. During the time when the eggs are hatching, they keep the interior of the hive at a very constant temperature of 35° (Hess, Himmer). The observed variations often lie within half a degree, that is to say, bees regulate the warmth of the hive just as exactly as mammals regulate the temperature of the blood.
It goes without saying that they must possess sensory cells by means of which they perceive heat. They raise the temperature of the hive apparently by muscular movements; over-heating they counteract by carrying in water and allowing it to evaporate.
Nocturnal Lepidoptera regulate the temperature of the body before flying. As a result of the heat produced by muscular activ ity, they attain, during flight, a temperature of about 36-40°. Before they fly off, they make very rapid, vibratory movements with their wings, buzzing or whirring. This, as Dotterweich has recently proved, is simply to raise the temperature of the body. As soon as the flight temperature is reached, they fly off. Fig. 11 represents, graphically, the raising of the body temperature during this vibration.
Very little is known, up to the present time, of heat perception in the rest of the Invertebrata. (W. v. BUD.)
For the sensory function of touch in man see SKIN, SENSORY FUNCTIONS OF.