one-celled body, without organs, and yet with its capacity for performing the necessary life processes, there are no special senses except one (perhaps two). The Amaba can feel. It possesses the tactile sense. And there are no special sense organs except one, which is the whole of the outer surface of the body. If the Amaba be touched with a fine point it feels the touch, for the soft viscous protoplasm of its body flows slowly away from the foreign object. The sense of feeling or touch, the tactile sense, is the simplest or most primitive of the special senses, and the simplest, most primitive organ of special sense is the outer surface or skin of the body. Among those simple animals that possess the simplest organs of hearing and perceiving light, we shall find these organs to be simply specialized parts of the skin or outer cell layer of the body, and it is a fact that all the special sense organs of all animals are derived or developed from the outer cell layer, ectoblast, of the embryo. This is true also of the whole nervous system, the brain and spinal cord of the vertebrates, and the ganglia and nerve commissures of the invertebrates. And while in the higher animals the nervous system lies underneath the surface of the body, in many of the lower, many-celled animals all the ganglia and nerves, all of the nervous system, lie on the outer surface of the body, being simply a specialized part of the skin.

119. The sense of touch.-In some of the lower, manycelled animals, as among the polyps, there are on the skin certain sense cells, either isolated or in small groups, which seem to be stimulated not alone by the touching of foreign substances, but also by warmth and light. They are not limited to a single special sense. They are the primitive or generalized organs of special sense, and can develop into specialized organs for any one of the special senses.

The simplest and most widespread of these special senses with, as a whole, the simplest organs, is the tactile

sense, or the sense of touch. The special organs of this sense are usually simple hairs or papillæ connecting with a nerve. These tactile hairs or papillæ may be distributed pretty evenly over most of the body, or may be mainly concentrated upon certain parts in crowded groups. Many of the lower animals have projecting parts, like the feeling tentacles of many marine invertebrates, or the antennæ (feelers) of crabs and insects, which are the special seat of the tactile organs. Among the vertebrates the tactile organs are either like those of the invertebrates, or are little sac-like bodies of connective tissue in which the end of a nerve is curiously folded and convoluted (Fig. 141). These little touch corpuscles simply lie in the cell layer of the skin, covered over thinly by the cuticle. Sometimes they are simply free, branched nerve-endings in the skin. These tactile corpuscles or free nerve-endings are especially abundant in those parts of the body which can be best used for feeling. In man the finger-tips are thus especially supplied; in certain tailed monkeys the tip of the tail, and in hogs the end of the snout. The difference in abundance of these tactile corpuscles of the skin can be readily shown by experiment.

n, nerve.

With a pair of compasses, whose FIG. 141.-Tactile papilla of points have been slightly blunted, touch the skin of the forearm of a person who has his eyes shut, with the points about three inches apart and in the direction of the length of the arm. The person touched will feel the points as two. Repeat the touching several times, gradually lessening the distance between the points. When the points are not more than an inch to an inch and a half apart, the person touched will feel but a single touch-that is, the touching

of both points will give the sensation of but a single contact. Repeat the experiment on the tip of the forefinger, and both points will be felt until the points are only about one tenth of an inch apart.

120. The sense of taste.-The sense of taste enables us to test in some degree the chemical constitution of substances which are taken into the mouth as food. We discriminate by the taste organs between good food and bad, well-tasting and ill-tasting. These organs are, with us and the other airbreathing animals, located in the mouth or on the mouth parts. They must be located so as to come into contact with the food, and it is also necessary that the food substance to be tasted be made liquid. This is accomplished by the fluids poured into the mouth from the salivary glands. With the lower aquatic animals it is not improbable that taste organs are situated on other parts of the body besides the mouth, and that taste is used not only to test food substances, but also to test the chemical character of the fluid medium in which they live.

The taste organs are much like the tactile organs, except that the special taste cell is exposed, so that small particles of the substance to be tasted can come into actual contact with it. The nerve-ending is usually in a small raised papilla or depressed pit. In the simplest animals there is no special organ of taste, and yet Amaba and other Protozoa show that they appreciate the chemical constitution of the liquid in which they lie. They taste-that is, test the chemical constitution of the substances-by means of their undifferentiated body surface. The taste organs are not always to be told from the organs of smell. Where an animal has a certain special seat of smell, like the nose of the higher animals, then the special sense organs of the mouth can be fairly assumed to be taste organs; but where the seat of both smell and taste is in the mouth or mouth parts, it is often impossible to distinguish between the two kinds of organs.

In mammals taste organs are situated on certain parts of the tongue, and have the form of rather large, low, broad papillæ, each bearing many small taste-buds (Fig. 142). In fishes similar papillæ and buds have been found in various places on the sur

face of the body, from

which it is believed that tb...

the sense of taste in

fishes is not limited to

the mouth. In insects

the taste-papillæ and taste-pits are grouped

in certain places on the FIG. 142.-Vertical section of large papilla on

mouth parts, being es

pecially abundant on

tongue of a calf; t. b., taste-buds. After LOVÉN.

the tips of small, segmented, feeler-like processes called palpi, which project from the under lip and from the socalled maxillæ.

121. The sense of smell.-Smelling and tasting are closely allied, the one testing substances dissolved, the other testing substances vaporized. The organs of the sense of smell are, like those of taste, simple nerve-endings in papillæ or pits. The substance to be smelled must, however, be in a very finely divided form; it must come to the organs of smell as a gas or vapor, and not, as to the organs of taste, in liquid condition. The organs of smell are situated usually on the head, but as the sense of smell is used not alone for the testing of food, but for many other purposes, the organs of smell are not, like those of taste, situated principally in or near the mouth. Smell is a special sense of much wider range of use than taste. By smell animals can discover food, avoid enemies, and find their mates. They can test the air they breathe as well as the food they eat. In the matter of the testing of food the senses of both taste and smell are constantly used, and are indeed intimately associated.

The sense of smell varies a great deal in its degree of development in various animals. With the strictly aquatic animals-and these include most of the lower invertebrates, as the polyps, the star-fishes, sea-urchins, and most of the worms and mollusks-the sense of smell is probably but little developed. There is little opportunity for a gas or vapor to come to these animals, and only as a gas or vapor can a substance be smelled. With these animals the sense of taste must take the place of the olfactory sense. But among the insects, mostly terrestrial animals, there is an extraordinary development of the sense of smell. It is indeed probably their principal special sense. Insects must depend on smell far more than on sight or hearing for the discovery of food, for becoming aware of the presence of their enemies and of the proximity of their mates and companions. The organs of smell of insects are situated principally on the antennæ or feelers, a single pair of which is borne on the head of every insect (Fig. 143). That many insects have an extraordinarily keen sense of smell has been shown by numerous experiments, and is constantly proved by well-known habits. If a small bit of decaying flesh be inclosed in a box so that it is wholly concealed, it will nevertheless soon eating beetle, showing be found by the flies and carrion beetles that either feed on carrion or must always lay their eggs in decaying matter so that their carrion-eating larvæ may be provided with food. It is believed that ants find their way back to their nests by the sense of smell, and that they can recognize by scent among hundreds of individuals taken from various communities the members of their

FIG. 143.-Antenna of a leaf

smelling-pits on the expanded terminal segments.