Single epidermal cells or groups of such cells are specialized for the reception of stimuli leading to sensations of touch, but in such forms as Coelenterates and Annelids many scattered cells of the kind probably minister to other senses besides that of touch. And it must be remembered that even the special sensecells of hearing and sight are derived from the skin, which is in fact the primitive sense-organ. Cells which are regarded as tactile, from some of the lowest animals, are represented in fig.

Fig. 1030.

1029.

The firm external covering with which the bodies of Arthropods are clothed is naturally a hindrance to the

greatly enlarged. On the right is a group of reception of stimuli by the underlying

such structures, and on the left a single one,

in section.

hard investment.

epidermis. The difficulty is got over by the existence of little pores in the Under each pore is an enlarged sense-cell, placed at the base of a stiff tactile bristle, with which external bodies come into contact (fig. 1030).

In aquatic Vertebrates the sense-cells of the skin are in direct contact with the surrounding medium, although they are not

Ep.

T.C

N.F.

A

Sh

B

Fig. 1031.-Organs of Touch

infrequently protected by being lodged in pits, grooves, or canals S.c. which open at intervals to the exterior. But in terrestrial Vertebrates there are special end-organs of touch which have sunk below the epidermis, though they remain sufficiently near to the surface to be stimulated when the body comes into contact with surrounding objects. Such are the groups of touch-corpuscles which are to be found in the skin of the Frog, and around the edge of the Duck's bill (fig. 1031). The latter animal feeds upon small worms, &c., which live in the mud that is strained through its bill, and such special arrangements are clearly necessary to aid in the discrimination between what is edible and what is not. Another example is afforded by the

A, Small piece of the skin of a Frog, in vertical section, enlarged; Ep., epidermis; T.C., touch-corpuscles; N.F., nerve-fibres. B, Touch-corpuscle from the bill of a Duck, much enlarged; S. C., sensecells; N.F., nerve-fibre; Sh., fibrous sheath.

numerous touch-corpuscles which underlie the little ridges seen. on the tips of our fingers and thumbs (fig. 1032). In Reptiles, Birds, and Mammals there are also curious struc

tures known as Pacinian bodies (fig. 1033), in which the ending of a nerve is surrounded by a series of layers arranged almost like the coats of an onion. There is reason to think that these are very sensitive to slight pressures. They abound, for instance, in the wing-membranes of Bats, and it is well known that these creatures can easily steer their way in the dark through a veritable maze of obstacles, such as that afforded by a series of strings running in various directions. Pacinian bodies are also found connected with tendons, ligaments, and various internal organs. The use of these is probably to apprise the central nervous system of variations in pressure and tension which take place as between the different parts of the body

itself.

We are still very much in the dark as to how far there do or do not exist special end-organs which are affected by variations in temperature. It is known that definite spots in the human skin are sensitive to such variations, but there do not appear to be any special sense-organs in these spots. Some of the sensory nerve-fibres terminate in the skin by dividing into a number of little branches which do not become continuous with modified epidermal cells, and it has been suggested that these "free nerve endings" are related to the temperature sense.

Fig. 1032. TouchCorpuscle from Finger Tip of Man, in section, greatly enlarged

Fig. 1033.-A Pacinian Corpuscle in Longitudinal Section, enlarged. A nerve-fibre (nn), surrounded by a sheath (), enters the base of the corpuscle, loses its sheath, traverses a central core (m), and ends in an irregular expansion (a). The cor

While the entire external surface of the body is sensitive to contact, pressure, and changes in temperature, this is in many cases insufficient to enable the requisite puscle is mostly made up of numerous adjustments to the environment to be brought about. And we accordingly find that in many animals. organs of active touch have been evolved, which explore the

surroundings, and help to detect the presence of food, or to give warning of danger. Such are the tentacles of Jelly-Fish and Sea-Anemones, the slender outgrowths on the head of a SeaCentipede, the two pairs of antennæ on the head of a Crayfish, the single pair on the head of an Insect, and the tentacles on the heads of Snails and Slugs. The "whiskers" of a Cat or Rabbit belong to the same class of structures. They are stiff

Fig. 1034.-A Deep-Sea Fish (Eretmophorus) with its Pelvic Fins drawn out into long Tactile Organs

hairs, at the base of each of which a touch-corpuscle is to be found. Such organs of active touch may either from the first have done duty as sensory organs, or may have originally been evolved in the interests of some other function. The former is probably true for the feelers of a Sea-Centipede or Insect, but the large feelers of a Crayfish (and very likely the small ones too) were probably jaws at an earlier stage, having later on been shifted in front of the mouth, and modified in shape and structure to do duty as sense-organs. There can be no doubt that the paired fins of Fishes were originally evolved in

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relation to swimming, but it sometimes happens that they have been transformed into tactile organs, as in the deep-sea form (Eretmophorus) represented in fig. 1034. Snakes employ their tongues as tactile organs, as

also do Woodpeckers and Anteaters. This, however, is probably only an extension of the original duties, for the primary use of the tongue seems to be that of a tactile organ in relation to the mouth-cavity.

TASTE

T.C

B

Fig. 1035.-Taste-Organs of a Wasp. A, Under side of left maxilla, enlarged, showing group of taste-cups (T.C.). B, A single taste-cup, greatly enlarged.

Sensations of Taste are primarily important because they assist in the selection of suitable food. The stimulus is a chemical one, and consists of substances in solution. We know but little about the gustatory organs of lower forms, but as these show a preference for certain kinds of food it is probably correct to assume that such organs are present. In the Earth-Worm, for example, groups of modified epidermal cells in the neighbourhood of the mouth are very

likely related to taste.

Certain regions of the mouth-parts of some Insects are studded with minute pits, beneath each of which is a sense-cell, drawn out externally into a short bristle, and continuous with a nerve-fibre internally. They are present, for example, in Bees and Wasps, and are almost certainly of a gustatory nature (fig. 1035).

Cuttle-Fishes and many Snails possess a sense-organ on the floor of the pharynx, below the front end of the rasping-ribbon. It probably has to do with taste.

Fig. 1036.-Taste-Bud from the Tongue of a Rabbit, in longitudinal section, greatly enlarged. The bud contains slender taste-cells, the external ends of which project into a little pit continuous with the mouth-cavity.

In Lung-Fishes, Amphibians, Reptiles, Birds, and Mammals the organs of taste consist of groups of sense-cells in the lining of the mouth-cavity, and since this cavity is developed as an

in-pushing from the exterior the cells in question are of ectodermic nature. The largest amount of specialization takes place in Mammals, where the "taste-buds ", as the group of cells are called, are associated with small projections or papillæ of the surface of the tongue (fig. 1036).

SMELL

Many of the lower animals can undoubtedly smel as well as taste, though to definitely associate this sense with special cells or groups of cells is not at present possible. Our knowledge is more complete in the case of Arthropods, Molluscs, and Vertebrates, where experiments lead to results of more de

Fig. 1037.-Olfactory Organs. A, A small part of the outer branch of antennule of a Crayfish is shown at a, with groups of olfactory setæ on its under side, enlarged; b, an olfactory seta, further enlarged. B, Tip of feeler of a Millipede, greatly enlarged, showing olfactory cylinders among the ordinary tactile bristles. c, Two olfactory cones from feeler of a Wasp, in section, greatly enlarged.

finite kind. In all cases the stimulus is of a gaseous nature, and in aquatic animals the gases that are smelt are dissolved in the surrounding water. The sense of smell is obviously of great importance as regards adjustment to the environment. By its means food is in many cases detected, while it often enables animals to recognize friends or foes, even when these are at a considerable distance. This is, of course, due to the nature of the stimulus. Since Smell, Hearing, Sight, and the Temperature Sense are able to give information about objects which are more or less far away, they may be grouped together as Distance-Senses (telæsthetic senses), and are in marked contrast to Touch (so far as haptic sensations are concerned), which only conveys knowledge regarding things that actually come into contact with the skin.