THE LIFE OF A CELL.

3

digest; they grow and multiply; they move and feel. Their perceptions, indeed, are no doubt confused and undifferentiated, and perhaps devoid of consciousness. The soft protoplasm of which they consist is dimly affected by external stimuli, as, for instance, by the waves of light or of sound. These forms, however, are all minute, and, indeed, almost invisible to the naked eye. The larger animals are built up of a number of cells.

Let us, then, consider the possible modes in which an organ of sense, say an eye, may have originated.

In the simpler forms, the whole surface is more or less sensitive. Suppose, however, some solid and opaque particles of pigment deposited in certain cells of the skin

с

h

Fig. 1.-Diagram of skin. c, Cuticle; h, cellular or hypodermic layer.

(Fig. 1). Their opacity would arrest and absorb the light, thus increasing its effect, while their solidity would enhance the effect of the external stimulus. A further

C

h

Fig. 2.-Diagram of skin. c, Cuticle; h, cellular or hypodermic layer.

step might be a depression in the skin at this point, which would serve somewhat to protect these differentiated and more sensitive cells, while the deeper this depression the greater would be the protection.

The epithelial cells frequently secrete more or less matter, which may form a more or less solid ball. This might be set in vibration by the sound-waves, and would thus increase the effect on the epithelial

4

POSSIBLE MODES OF ORIGIN.

cells. Such a body is known as an otolithe. On the other hand, it might serve as a lens, and by condensing

с

h

n

Fig. 3.-Diagram of origin of a sense-organ. c, Cuticle; h, hypoderm ; * n, nerve.

the light would act like a burning-glass, and increase its effect on the cells below. A further stage would be

с

1

h

n

Fig. 4. Diagram of further stage in the origin of a sense-organ,

that the immediately subjacent cells, acted on by the increased stimulus, might (Figs. 3 and 4) develop into special nerve-tissue.

*I.e. the cellular layer below the cuticle.

OF SENSE-ORGANS.

5

Nor is this a merely imaginary case. Each of the above stages may be found in actual existence-that, for instance, indicated in Fig. 2 in the limpet (Fig. 92); Fig. 3, in Trochus (Fig. 93); and Fig. 4 in the snail, Helix or Murex (Figs. 94, 95). Recent researches indicate that the eyes of Articulata (insects, etc.) have, in some cases at least, a similar history. But more than this, if the development of the eye of an individual snail be watched in the egg, it will be found to pass successively through stages resembling Fig. 2, then Fig. 3, and then Fig. 4.

In other cases, however, the organs of sense have a different origin and history. Suppose, for instance,

C

h

Fig. 5.-Diagram of origin of a sense-organ.

that the hypodermic layer were at any spot (Fig. 5) somewhat more strongly developed than elsewhere; in that case, the cuticle secreted by the hypodermic cells would tend to be rather thicker than usual. This would again (Fig. 6) constitute a lens, and serve to condense the light. That certain eyes have actually arisen in this way is indicated by Fig. 7, representing a section

Fig. 6.-Diagram of further stage in the origin of a sense-organ.

through the eye of the larva of a water-beetle (Dytiscus). Nor, as we shall presently see, do these two types of development by any means exhaust the ways in which eyes may originate. In the two cases given the eyes originate from the skin, but in othersfor instance, in ourselves—the percipient elements are formed from the central nervous system.

6

ORIGIN OF EYE AND EAR.

The tissues of the lowest animals have not been shown

h

to contain any special nerve-fibres, but underneath those parts of the surface where, either in the manner indicated above, or in some other, the effects of external stimuli are heightened by any structural modifications, there would be a tendency to the speci

Fig. 7.-Section through the simple eye of a alization of an excep

young Dytiscus larva (after Grenacher). h,

modified hypodermic cells; r, retina.

Hypoderm; 1, lens; o, optic nerve; 9, P, tionally sensitive tissue. Moreover, such an

organ as that represented in Fig. 4 might serve either as a rudimentary ear or an eye. It might, indeed, be acted on by the waves both of light and of sound. Such organs-as, for instance, in the case of marginal bodies round the edge of certain jelly-fishes (Medusa; see Figs. 8 and 50)-have been regarded by some naturalists

*

as eyes, and by others as ears. Haeckel suggests that some may be warmth-organs. Fig. 8 represents one of the marginal sense-organs of a Medusa (Ontochis), where we have a row of brilliantly re

Fig. 8.-Auditory vesicle of Onto- fractive spherules, which from analogy are considered to serve as otoliths; but which, under other circumstances, might be, and in fact have been by some, regarded as the lenses of a simply constructed organ of vision.

* 66

Report on Deep Sea Medusa," "Challenger Reports," vol. iv.

THE SENSE OF TOUCH.

7

Even among the most highly specialized organs of sense, it is impossible not to be struck by the similarity between the cones in the retina (Fig. 79) and certain organs in the antennæ of insects (Fig. 42) which are generally considered as olfactory. It does not follow that an organ with a nerve, a lenticular body, and pigment, should necessarily be an eye. Nor, on the other hand, is there anything in the structure of the organs, for instance, of smell or taste which throws any light on the perceptions we receive from them. That there should be separate nerve-fibrils in our own skin, not only for the sensations of temperature and of touch, but, as appears from the researches of Blix and Goldschneider, even of heat and of cold, we had not anticipated à priori; and it would be difficult to prove in any animal but ourselves.

THE SENSE OF TOUCH.

I commence with the sense of touch, as being the one which is most generally distributed, and from which the others appear to have been in some cases developed. The senses are not, indeed, as already mentioned, always to be easily distinguished from one another; and it would seem that the same nerve may be capable of carrying different sensations according to the structure of the end organs.

The sensibility of our skin appears to be mainly due to a plexus of fine nerve-fibres, which end in free terminations between the cells of the skin (rete mucosum). There are also in some parts of the skin two sets of minute corpuscles, which are called after their discoverers, the first Vaterian, or more commonly Pacinian, corpuscles; the second, Meissner's or Wagner's corpuscles.