Popular Science Monthly/Volume 9/June 1876/Organisms and their Media
HEAT and light are physical influences to which even the lowest units of living matter respond, whether their mode of life and nutrition is most akin to that of plants or to that of animals. These influences act on such organisms, either by stimulating, retarding, or otherwise modifying the chemical changes naturally occurring in their interior, and upon the existence of which their life depends. Where the vital processes of the organism are stimulated by these physical agencies, their incidence may, in many instances, become the cause of so-called "spontaneous movements." The term, however, as applied to movements, is a bad one—since all the movements of an organism are alike dependent upon a series of antecedent states of contractile and other tissues. There is some sort of foundation, it is true, for the popular mode of expression. A movement is not said to be "spontaneous" if it follows immediately upon some external impression as a cause; the term is generally applied where the cause of the movement is not distinctly recognizable. In some instances the undetected or unconsidered external cause may be the incidence of a diffused physical agent such as heat, which, by stimulating the vital processes, seems to give rise to spontaneous movements. In other cases so-called "spontaneous" movements are to be referred to internal states or changes, whose origin is even less distinctly traceable, to impressions, it may be, which emanate from some of the internal organs, and thence are transmitted to ganglia in direct relation with some of the organs of locomotion.
Heat mostly acts on organisms upon all sides alike, so that, though it may stimulate their life-processes generally, and, in some instances, give rise to movements, these movements are not determined in one, more than in another, direction. Thus, while heat stimulates the "to-and-fro" or the gyratory movements of bacteria, and also renders more striking and rapid those changes of form which all amœboid organisms are apt to display, the movements evoked are random, and apparently devoid of all purpose.
It is not altogether similar, however, with the influence of light. This agent almost always, and necessarily, falls more on one side than on another; and consequently it often suffices to induce movements to be made in definite directions, by the lower forms of life, just as it causes definite and responsive movements to be executed by certain parts of higher plants, which come fully under its influence. In each case the movement, or altered position, is due to some nutritive change; that is, to some alteration, whatever its nature, in the activity of the life-processes taking place in the part impressed by the light. So that, whether we have to do with the movement of a sunflower, or with the locomotions of minute living units, the essential mode of production of the movement is probably similar. Of the actual locomotions of minute living units under the influence of light many instances might be cited; it will suffice, however, to mention the fact that any green zoöspores which may have been uniformly diffused through the water are very apt, when the vessel containing them is placed near a window, to collect on the surface of the water at the part where most light fulls upon them. Minute animal organisms are, however, often affected quite differently by this agent. They are frequently caused to move away from, rather than toward, its source; so that the creatures thus impressed "seek" the shade rather than the glare of sunlight.
The action of such influences and the production of such movements form the beginnings or substrata, as it were, of other phenomena with which we are now more particularly concerned. The unilateral influence of light and the movements to or from its source to which it may give rise afford a connecting link between diffused causes like heat, which, by affecting the general activity of the vital processes in the organisms, may lead to purely random movements, and those more localized influences now to be considered, to which the various definite or responsive movements of organisms are attributable.
The first, because it is the simplest, of these localized influences to be considered is a shock or mechanical impact of some kind, falling upon the external surface of the organism. This is the primordial or most general of all the modes by which the surface of an organism is impressible, and its sensitivity to such stimuli is both in the stage of impression and the stage of reaction closely akin to the general organic irritability of protoplasm—which, indeed, unquestionably constitutes its starting-point. This mode of impression, moreover, is one which tends to establish a correspondence between the organism and the most common events or properties of the medium in which it lives and moves. It is consequently the mode of impressibility most extensively called into play among all the lower forms of animal life. And although the whole, surface of an organism, or the greater part of it, in one of the simple animals to which we are referring, may be more or less impressible to shocks or impacts from contact with surrounding bodies, it often happens that such impressions more frequently fall upon, and are more readily received by, certain appendages situated at the anterior extremity of the animal, in close proximity to the mouth. Such specialized parts or tactile appendages are known as papillæ, setæ, tentacles, antennæ, or palpi, according to the forms which they assume in different animals.
Why such organs are developed so frequently at the anterior extremity of the animal, and in the neighborhood of the mouth rather than on other parts of the body, is not difficult to explain. Whatever the mode by which they are evoked or called into being (and the most opposite views may be entertained upon this subject), it seems obvious that, if organs of this kind are to be present at all, they should occur in situations where they might be put to most use. In an animal accustomed to active locomotions, the mouth is, with only a very few exceptions, situated on that part of the body which is habitually directed forward. The anterior extremity thus comes to be the part of the body which is brought most into converse with its environment; and, of the diverse objects impinging against it, or against which it impinges, some are of a nature to serve the organism as food, and some are not. A higher degree of impressibility springs up, therefore, in this situation, where the parts are necessarily exercised so largely with impressions corresponding with food and with others having an opposite relation. It should not surprise us, therefore, to find among the lower animals that the most specialized tactile organs are found in the immediate neighborhood of the mouth. Such organs may be, and are in fact, not unfrequently both tactile and prehensile; though this is more especially the case in sedentary forms of life, like the hydra, the sea-anemone, or some of the tentaculated worms. The tentaculæ of the latter animals would seem to be possessed of an extremely high degree of impressibility, if we are to judge by the report of one who devoted much attention to the study of this class of organisms—the late Dr. Williams, of Swansea. He says: "It is not easy, for those who have never enjoyed the spectacle of the 'feat of touch' performed by the tentaculated worms, to estimate adequately the extreme acuteness of the sensibility which resides at the extremities of the living threads with which the head and sides of the body are garnished. They select, reject, move toward, and recede from, minute external objects with all the precision of microscopic animals gifted with the surest eagle-sight."
But it often happens that the solid bodies serving as food are in a measure soluble, so that, in animal organisms comparatively low in the scale of complexity, some of the tactile structures within or around the mouth may undergo a further specialization by which they become able to discriminate and respond to impressions of a slightly different nature. These parts become sensitive to a more refined kind of contact, such as may be yielded by certain dissolved elements of the food substance, whose local action may be attended by some slight chemical change in the tissue of the organ. Impressions are thus produced whereby the "sapidity" or flavor of bodies is appreciated, and such impressions gradually become associated with definite related movements.
No distinct organ of "taste" or specialized gustatory surface is known to occur among invertebrate animals, except in insects and in such higher mollusca as gasteropods and cephalopods; although such a mode of impressibility does, doubtless, exist in many other of the lower forms of life. Impressions of the two orders already referred to—more or less distinct from one another—are those by which alone multitudes of the lowest forms of animal life, such as polyps, medusæ and various kinds of worms, appear to hold converse with the outside world. Touches and tastes are the names which we apply to the subjective effects of such impressions; and, though it is impossible at present wholly to ignore this point of view, or to use language which is not colored by it, I do not now wish to say anything with regard to the nature or intensity of the feelings that may be associated with corresponding impressions in the lower animals. The reader must for the present look rather to the objective effects of these impressions, and in so doing he will learn that these become organically associated with a nervous mechanism by whose intermediation they are able to evoke distinctive movements of a responsive nature.
Seeing, however, that tactile and gustatory impressions can only be made by actual contact of external bodies with the specialized parts of an organism, such impressions are not of a kind to excite movements in quest of food; although they may lead to correlated movements of parts adjacent to those which are touched, as when all the tentacles of a sea-anemone close round a body that has come into contact with some one of them. This effect is due to a radiation of the primitive stimulus, and we may see in such a set of actions only a more rapid and slightly more complex result than is known to follow the irritation of one of the peripheral tentacles on the leaf of a sun-dew. In the latter case the bending of the tentacle actually irritated is slowly followed by the bending of others under the influence of an internally diffused stimulus.
Movements in actual quest of food may, however, be excited in other animal organisms by impressions which suffice to bring them into relation with more or less distant bodies. The way is paved for this result when some portion of the anterior and upper surface of the animal, in which aggregations of pigment occur, becomes more than usually sensitive to light. A dark body passing in front of such a region gives rise to certain molecular changes therein, and these molecular changes differing among themselves become capable of exciting distinctive impressions in the organism which it gradually becomes attuned to discriminate. The power of discrimination in this, as in all other cases, is indicated by the organism's capability of responding to impressions by definite muscular movements as when the oyster, with the valves of its shell apart, instantly closes them if a shadow is projected over certain sensitive pigment-specks or so-called "eyes" at the edge of its mantle.
This beginning of visual impressions truly enough shows itself as a merely exalted appreciation of tactile impressions; and, inasmuch as such an appreciation of the presence of near bodies would in so many instances be quickly followed by a more gross mechanical contact, the rudimentary visual impression is, as Spencer says, a kind of "anticipatory touch." From this simple beginning, in which bodies only slightly separated from the impressible foci excite certain general or only vaguely specialized impressions corresponding to light and shade therein, the organs of sight and their impressibility gradually become more and more elaborate. To rudimentary aggregations of pigment transparent media are added, which condense the light on these impressible patches, and these media in other organisms are sufficiently like a lens to be adequate to form a definite image of an external body on the layer of pigment, which, on its other side, is in contact with a nerve-expansion communicating with a contiguous ganglion. Numerous simple structures of this kind may exist apart from one another, as in many bivalve mollusks, or they may be far more numerous and closely aggregated so as to form such compound eyes as are met with in crustaceans and in insects. Or individual ocelli may be perfected, as in spiders, or lower crustacea, though most notably of all among the cuttle-fish tribe in which two movable eyes are met with, whose organization is just as perfect as that of the eyes of fishes.
The difference in degree and range of sensitiveness existing between the simple "eye-specks" of some of the lower worms and the elaborate organs existing in the highest insects and mollusks is enormous. The range and keenness of vision become progressively extended, so that creatures with more perfect eyes are capable of receiving and appreciating impressions from objects more and more distant, and the various actions which become established in response to impressions habitually made upon such sensitive surfaces increase enormously in number, variety, and complexity. The relation existing between the keenness of the sense of sight and the powers of locomotion of insects has long been recognized by naturalists. Prof. Owen, for instance, thus alludes to it: "The high degree in which the power of discerning distant objects is enjoyed by the flying insects corresponds with their great power of traversing space. The few exceptional cases of blind insects are all apterous, and often peculiar to the female sex, as in the glow-worm, cochineal-insect, and parasitic stylops."
The various actions of insects and of invertebrate animals generally are, however, found to be easily capable of classification. They are, in the main, subservient to the pursuit and capture of prey, to the avoidance of enemies, to the union of the sexes, or to the care of their young. To such ends are their various motions, whether occasional or habitual, more or less directly related. Nothing is here said, however, as to the extent to which such ends are realized by the animals themselves.
In vision, as I have said, we have to do with a refinement of the sense of touch, whereby the animal becomes sensible of impressions produced by "waves" of light emanating from a distance, and is thus brought into mediate contact with certain distant objects. A refinement of the organs of taste may also occur whereby bodies possessing sapid qualities are capable of impressing organisms still at a distance. Just as vision, in fact, is, in its most elementary phases, a sort of "anticipatory touch," so is smell a kind of anticipatory taste. Yet the two cases are not altogether similar. In vision, the contact—if it may be so termed—with the distant body is mediate, through the intervention of ethereal undulations; while in smell we have to do with a case of immediate contact, not with the distant body itself of course, but with extremely minute particles which it gives off on all sides. An "emission" theory serves to explain the diffusion of odors, though it will not hold for the diffusion of light. From what I have said it may be inferred that, as regards the delicacy of their respective physical causes, the sense of smell occupies an intermediate position between taste and sight.
It is regarded as a matter of certainty by naturalists that such creatures as spiders, crustacea, insects, and the higher mollusks, are capable of being impressed in some way by odors, and that their actions are to a certain extent regulated by such impressions. We have, however, no definite knowledge concerning the parts of the surface which in these, and perhaps in still lower organisms, are attuned to receive such influences. Although a rudimentary sense of smell seems unquestionably to be possessed by such aquatic forms of the invertebrata as Crustacea and the higher mollusks, it is, perhaps, a sense-endowment which generally exists in a more developed and more varied form among air-breathing animals. In whatever forms of life it may be met with, however, the sense of smell seems to be very largely indeed related to the detection and capture of food; so, that, in these relations, it comes to the aid of the already-existing senses of sight, touch, and taste, though it has the peculiarity of being scarcely otherwise called into activity among the invertebrata.
Although we have no positive knowledge concerning the situation of the organs of smell among invertebrate animals, there is good reason for believing that in crustacea they are to be found at the base of the antennules; that in cephalopods they are represented by two little fossæ in the neighborhood of the eyes; and that in insects a power of appreciating odors is possibly possessed either by the antennae themselves, or by a pair of fossæ near their bases. Another cephalic organ has also been referred to as possibly endowed with a power of being impressed by odors. Thus Owen says: "The application, by the common house-fly, of the sheath of its proboscis to particles of solid or liquid food, before it imbibes them, is an action closely analogous to the scenting of food by the nose in higher animals; and, as it is by the odorous qualities, much more than by the form of the surface, that we judge of the fitness of substances for food, it is more reasonable to conclude that, in this well-known action of our commonest insect, it is scenting, not feeling, the drop of milk or grain of sugar."
Looking to the importance of this endowment in reference to the perception of food, and also looking to the situation of the organs of smell in all the vertebrate animals, there is good reason for believing that any similar organs of sense which may exist among invertebrate organisms would be found in close proximity to the mouth, so as to permit of that joint or associated activity between the sense of taste and the sense of smell which is met with in all higher forms of life.
As already pointed out, there are also obvious reasons why the principal specialized tactile organs that may present themselves in lower animals should be found in the neighborhood of the mouth; and for similar reasons, if for no other, the anterior extremity of the body, or the upper surface near this anterior extremity, is the site in which visual organs might be used with most advantage by their possessor. To an active animal, visual organs would not only be more useful at the anterior extremity of the body than elsewhere in relation to its food-taking movements, but also in reference to all other uses to which such appendages may be applied during active locomotions from place to place. To this situation of the eyes only two or three exceptions are met with among animals endowed with powers of locomotion, and these deviations are explicable by reference to the habits and modes of life of the organisms in question.
The part of the body bearing the mouth, and the various sensory organs already named, is familiar to all as the "head" of the animal; and it is owing to the fact of the clustering of sense-organs on this part of the body that the head contains internally a number of nerve-ganglia in connection therewith. This aggregate mass of ganglia constitutes the brain of the invertebrate animals, which, as we shall find, differs much in different classes of animals, not only in disposition and in size, but also in respect to the relative proportion of its component parts. The size of the respective ganglia, indeed, necessarily varies in accordance with the relative importance and complexity of the several sense-endowments already mentioned—those of touch, taste, smell, and vision. The ganglia thus constituting the brain of invertebrate animals are not only connected with their own particular external organs, but, in addition, we find the several ganglia of the two sides brought into relation among themselves and with their fellows by means of connecting fibres, while they are also more distantly united with other nerve-ganglia in different parts of the body by means of commissural fibres.
But another special sense-endowment remains to be referred to. This has to do with the organism's power of appreciating sounds or "auditory" expressions—a power which is, however, probably possessed in only a low degree by most invertebrate animals; since, even in the most perfect form of the organ of hearing among them we have to do with a very rudimentary structure. In this respect there is a great difference between the sense of sight and the sense of hearing. While the eye of the cuttle-fish attains a degree of elaboration that does not fall so very far short of the most perfect form which it displays among vertebrate animals, the organ of hearing, as a mere organ, in all forms of the invertebrata is remarkable for its simplicity, and remains notably inferior to the highest type attained by this sensorial apparatus—which, with its nerve-connections, becomes so enormously developed in many mammals and in man.
Like the sense of sight and the sense of smell, that of hearing, even in its simplest grades, serves to bring the organism into relation with more or less distant bodies, so long as they are sufficiently sonorous to transmit the so-called "sound" vibrations through water or air to the sensitive organs which become attuned to receive such impressions.
An auditory organ does not seem to be present at all—certainly none has as yet been detected or inferred to exist—in many of the lower forms of life; while in other animals, though inferred to exist, it remains as yet unrecognized. This is the case, for instance, with the majority of crustacea, spiders, and insects. Judging from the instances in which an organ of hearing has been detected in mollusks, and in a very few representatives of the classes above named, it seems (however novel the information may be to many readers) that it is an organ of special sense which is not habitually, or even usually, found in the head, and in direct relation with one of the ganglia composing the brain. Further remarks, however, on this subject must be deferred until a brief description has been given of the nature and distribution of the nervous system in some of the principal groups of invertebrate animals.
These, then, are the commonly-received modes by which organisms are impressed from without, and by which they attune themselves to the conditions and actions in their medium. It was recognized by Democritus, and other ancient writers, that they are all of them derivatives, or more specialized modes of a primordial common sensibility, such as is possessed by the entire outer surface of the organism. Touch, taste, smell, vision, and hearing, are sense-endowments, having their origin in organs formed by a gradual differentiation of certain portions of the external or surface layer of the body—that is, of the part in which common sensibility is most frequently called into play. And just as this common sensibility is a crude or general sense of touch, so are the several special senses only more or less highly-refined modes of the same sense-endowment. In the case of special tactile organs, of organs of taste and organs of smell, the several contacts between the animal and the body which impresses it, though differing in their delicacy or refinement, are still immediate; while in the case of the organs of hearing and the organs of vision the contact between the sensitive surfaces and the impressing body is mediate, by the intervention in the one case of vibrations transmitted through water or air, and, in the other, of vibrations from the often far-distant luminous body, through an intermediate and all-pervading ether.
The movements of locomotion, or of parts of the organism which become established in correspondence with these various impressions, slowly increase in number, definiteness, and complexity. Such responsive movements, however, are found, as a general rule, to have the effect of prolonging the action of any influences which previous individual or race experiences have proved to be favorable to the life and well-being of the organism; and, on the other hand, of cutting short or avoiding influences which past individual or race experiences have proved to be contrary to its general well-being. The capture and swallowing of food are ends to which a very large proportion indeed of the definite motions of most of the lower organisms are directed; and this direction of their energies is only a special case to be included under the rule above indicated; just as efforts to escape from predatory neighbors are other, though opposite, instances of the same rule.
In addition to the various modes of impressibility by external influence which we have hitherto been considering, there are certain internal modes of impressibility due to changes in the condition of internal parts of the organism. These are commonly spoken of as divisible into two categories: 1. The impressions derivable from, or in some way attendant upon, the contractions of muscles; and, 2. Impressions emanating from one or other of the various sets of internal organs, such as the alimentary canal and its appendages, the respiratory organs, the genital organs, or other internal parts.
With the first set of impressions we have at present nothing to do. They differ altogether from others, whether of external or internal origin, by the fact that they follow or accompany movements whose intensity they are supposed to measure, and do not themselves lead to movements. Granting that such impressions may have a real existence, it is obvious we can know nothing about them among invertebrate animals, if they have only a subjective existence, and do not cause an efflux of molecular movements along outgoing nerve-fibres.
The second category of internal impressions—those emanating from the viscera—are undoubtedly very important in relation to animal life generally. In part they have the effect of causing contractions of related muscular portions of the viscera—as when the presence of food in certain portions of the alimentary canal excites impressions, followed by contraction whereby the food is propelled farther on. In part, however, they act upon the principal nerve-ganglia—those constituting the brain—and thus excite the external sense-organs with which they are connected to a higher order of activity. Visceral impressions may cause an animal eagerly to pursue food, or to be alert in discovering its mate; so that in these, and in many other instances, internal impressions, reaching the cerebral ganglia, would seem to excite a higher receptivity to certain kinds of external impressions and a corresponding readiness to respond on the part of the moving organs whose activity is related to such external impressions.
- In some spiders the ocelli are situated rather far back on each side of the cephalo-thorax, but, as Siebold says: "The disposition and the direction of the organs are in relation with the mode of life of these animals, some of which wait for their prey hidden in chinks of a wall within silken tubes which they have constructed, while others hold themselves motionless in the centre of their webs, or wander from side to side, a mode of life which obliges them to look in all directions" ("Manuel d'Anatomie comparative," tome i., p. 308). According to Prof. Rolleston also in the crustacean genera Euphausia and Thysanopoda: "Eyes may be, contrary to the otherwise invariable rule in Arthropoda, found elsewhere than upon the head" ("Forms of Animal Life," p. cxxi.).