Part 17

The mere contiguity of parts will often affect the form of organisms. Thus, in many flowers parts which are adjacent become dwarfed, while others which are freely exposed become fully developed, as we see in the flowers of many _Umbelliferæ_, or plants of the parsley, fennel, and hemlock order.

The shapes of flowers bear relation (as we shall see later) to their need for attracting insects which by their visits effect the development of seed, and for repelling others the access of which would be hurtful.

The avoidance of enemies may be so effected by an organism that their access may be made impossible save in one direction, the extent of vulnerable surface even in that direction being minimized. We have an example of such a condition in those worms which live in calcareous tubes, and which are some of those called "tubicolous annelids."[60]

Again, the medium in which an organism lives--whether aërial or aqueous--has an important relation with its form. A delicate seaweed, the beautifully radiating form of which is a just object of admiration as long as it is supported by its denser natural medium (the sea water), collapses into an amorphous mass when withdrawn thence into the thin air. Obviously a much greater rigidity and strength of structure is needed to support an aërial organism than an aquatic one, unless the former can support itself on other solid structures, such as rocks or trees. In the latter case the form attained may be very elongated and slender, as in the many creeping and climbing plants, which are so often furnished with processes for grasping (tendrils) to aid them in their mode of life.

An aërial fixed organism, if it does not rise from the surface of the earth, cannot spread itself very far without developing other points of support--without rooting again. This re-rooting is a familiar phenomenon in many plants, as, _e.g._, the strawberry. But even a shrub like the common bramble (which is not itself prostrate, but which sends out extraordinarily prolonged branches) is aided by such a process. The ends of its long branches apply themselves to the ground and begin to pierce its surface, the incipient leaves of its terminal bud becoming metamorphosed into roots.

An aquatic fixed organism, however, may extend to a very great length, freely floating without effecting any such fresh attachment. Thus the seaweed _Laminaria digitata_[61] will spread over a circle 12 feet in diameter, while _L. longicornis_ grows in the form of an elongated riband, from 8 to 12 feet in length and 2 or 3 feet wide. The giant form _Macrocystis_ (with a much more subdivided outline) may extend to the extraordinary length of 700 feet.

The conditions under which needful gaseous interchange can be effected and food obtained by different living creatures, govern in various other ways the forms of their bodies.

Thus, if it is helpful to the life of a creature to submit as large a surface of its body as possible to the influence of light, or to the action of air or water, then for this purpose its body must be expanded and its expanded parts divided and subdivided as they extend in different directions. It is for this reason that trees branch, and that their branches and twigs divide and subdivide as they do. It is for this reason also that their branches do not grow out one above another in precisely the same direction, but, on the contrary, grow in such a manner that each one may overshadow those immediately beneath as little as may be. Similarly and for the same reason leaves are developed mostly in an alternating fashion, so that each may be able to expose its green surface to the light and air as much as possible.

Plant-like animals which grow up in an arborescent manner from a fixed base do not generally branch in so regularly alternating a mode as do plants, and in some cases their successive branches may even be regularly superimposed. This is due to their not requiring, as plants do, that their surface should be very extensively exposed to light, neither their gaseous interchange nor their nutrition being impaired by such superposition. The water which carries to them both the nutritious particles on which they feed and the gases they respire, will act with nearly or quite the same efficiency in either arrangement of their parts.

If the exigences of life require any organism to retain much fluid within it, this circumstance may lead to its assumption of a dilated more or less globular form, as in the melon cactus, and, to a less degree, in the leaves of the common stonecrop.

But the conditions under which alone certain fixed organisms can obtain their food may govern also their internal structure. Thus, we shall see that in plants which feed by absorbing matters through their roots, an internal arrangement has to be effected for distributing material thus obtained, and conveying it upwards through the stem. So, again, many fixed animals need a greater supply of food and gases than they can obtain from the water which bathes or may reach them without effort on their parts. Such animals may be provided with special internal structures, which cause currents of water to flow towards them, and very often to penetrate within them, as in the shell _Mya_ or the razor shell.[62]

Fixed subterranean creatures are rare, but such do exist, as, for example, the truffle (_Tuber cibarium_). Surrounding influences must in such instances be alike on all sides, while the imbedded position of such organisms render superfluous the development of any elongated process for the purpose of fixing them. Such creatures, then, have a spheroidal figure, and neither internally nor externally are their structures developed in special directions.[63]

The fixed organisms which are the most aërial in their habits are attached to elevated objects, such as trees, and necessarily have a portion of their frame set apart to fix them to the object which supports them. The most conspicuous creatures of this kind are, perhaps, the plants termed "Epiphytes," on account of this habit. Amongst them may be mentioned the beautiful orchids called "air plants," and the familiar mistletoe. Other vegetable organisms--the multitude of creeping plants--rear themselves to great heights by the aid of their more robust brothers, but they can hardly be reckoned as aërial organisms.[64]

The colours which plants display have sometimes a singular relation to the mountain elevations or geographical positions they inhabit, but these considerations will be aptly treated of in the relations borne by living creatures to physical conditions and to one another.

Living creatures which are capable of moving or being freely moved about, present us with similar but more marked differences.

Certain aquatic creatures drift passively about (borne by streams or currents) with no permanent relation between any fixed portion of their bodies and the medium which transports them. Such creatures being equally acted on on all sides by surrounding agencies might be expected (like the subterranean truffle) to exhibit a spheroidal figure, with only one kind of surface upon their whole exterior. This is just what we find to be the case in a variety of more or less minute organisms, such, _e.g._ as _Myxastrum radians_ and _Magosphæra planula_.[65]

The former of these consists, at one stage of its existence, of a small globular mass of protoplasm, from the whole periphery of which a multitude of fine pseudopodia radiate. When about to reproduce, the creature retracts its pseudopodia, and forms around its exterior a structureless coat or cyst, an action which takes place frequently in lowly organisms, and is called their process of _encystment_. The contents of the cyst then divides into separate bodies, which escape by the rupture of the cyst. Each of these bodies is enclosed in a silicious case with an aperture at one end, whence its contained protoplasm issues, and, having so issued, assumes a spherical shape.

_Magosphæra_ is another small creature which goes through a remarkable series of changes, the greater number of which exemplify the ball-like shape of body alike on all sides.

Wherever the surface of the body is covered by pseudopodia, those processes, inasmuch as they have a power of spontaneous movement, enable the creatures possessing them slightly to aid or to resist the drifting action of the water in which they float.

But a living organism may be devoid of any definite shape whatever, as in _Protamoeba_,[66] which consists of a mere particle of protoplasm, from which irregular-shaped processes of unequal size are irregularly protruded in every direction, so that the form of the creature may be said to be quite indeterminate.

The bodies of almost all organisms have, however, more or less definite forms, which may be all classed under seven morphological categories.

(1). The simplest form of all exemplifies _spherical symmetry_, and is that which we have seen in the truffle, the radiolarian, the volvox, _Myxastrum_ and _Magosphæra_. In this spherical form any number of axes drawn through the creature in any direction are equal.

(2). The next organic form is one in which the body sphere is more or less elongated at its poles, the latter being equal and similar. In such an organism we have one axis longer than any one of the others and central, while from this axis symmetrical radii can be drawn in all directions. This form may be said to exemplify _equipolar symmetry_, and such is found in some radiolarians, in some small parasites (_Gregarinida_),[67] and others.

(3). The next morphological category may be spoken of as _unipolar symmetry_. Bodies which exemplify it are like those included in the last category, save that the two poles of the body are not alike.

Instances of this symmetry are to be sought in creatures which have one end of their body fixed, or which always or mostly move with the same end of the body in front, and thus have their two extremities in more or less constantly different relations to surrounding influences.

The lowest worms and sponges may serve as examples of this symmetry in its simplest expression. As also may the curious compound tunicary called _Pyrosoma_.[68] In all such creatures the body does not extend out in the form of lateral prolongations.

But in many others it does send out processes on all sides, and in various directions, as in most trees and all plants which have a definite axis of growth, so that unipolar symmetry is the predominant symmetry in the vegetable kingdom.

(4). But unipolar symmetry with diverging outgrowths leads us to the next category which may be called _radial symmetry_. Under this head are included the forms of such creatures as possess unipolar bodies from which equal and corresponding outgrowths radiate in different directions.

We have examples of this in the starfishes, in the sea anemones, and in such plants as the melon cactus. But the outgrowths may project in only four directions, each being at right angles with the two neighbouring outgrowths. We thus get a crucial form of radiation, in which the body may be described as having one main axis (in the direction of motion) crossed by two other shorter but equal axes at right angles to it and to each other.

We have an example of this in _Tetraplatia volitans_,[69] an aquatic creature with an elongated body, which presents four distinguishable longitudinal surfaces, of which each opposite and corresponding pair is hardly distinguishable from one another.

(5). This form leads us directly to that kind of symmetry which is predominant in the animal kingdom and which is called _bilateral symmetry_. Forms of this kind exhibit four aspects which may be distinguished as right and left, dorsal and ventral. The body here presents a long axis (in the direction of motion) crossed by two shorter axes at right angles to it and to each other. Of these shorter axes, one connects the dorsal and ventral surfaces, while the other connects the lateral (right and left) surfaces, and these two axes may be, and generally are, unequal. All worms, insects, mollusks, fishes, birds, reptiles, and beasts, are examples of creatures with bilateral symmetry. The dorsal and ventral aspects of the body generally differ in correspondence with the different relations to surrounding conditions which they usually bear, as notably in snakes and creatures which glide with their bellies applied to the surface of the ground.

(6). The last kind of symmetry which here needs notice is that termed _serial symmetry_. In the creatures which exhibit it we have a body which is not only almost always bilaterally symmetrical but which is made up of a succession of similar parts, forming a series along its main or longitudinal axis. Insects, crabs, lobsters, and other allied forms give us examples of serial symmetry, but this is perhaps best seen in such animals as thousand legs and hundred legs--millipedes and centipedes.

Besides the fundamental distinctions which depend upon the kind of symmetry governing the form of any living being, other subordinate differences exist respectively related to the conditions under which the various activities necessary for life have to be carried on. Such activities are the needful gaseous interchange, the processes of reproduction, and the acquisition of food. Thus, the most intimate relation exists between the form of the body and the manner in which locomotion has to be effected, whether by the whole body or by processes projecting from it. If the latter, then whether by paddling or jumping; if by the whole body, then whether by lateral or vertical bendings of that body.

Thus, we see that fishes, which swim by lateral flexure of the body, have the tail expanded vertically; while in porpoises, which require vertical flexions (to come rapidly to the surface to breathe), the tail is expanded horizontally. On the other hand, creatures which swim not by either kind of body flexure, but by a paddling action only, have the tail shortened, as we see in swans and turtles. Further details of this kind will be more appropriately treated of in an Essay devoted exclusively to the consideration of the forms of animals.

There are a multitude of aquatic creatures which cannot be properly spoken of as either "fixed" or "mobile," for they are in fact both. They are creatures which move about by the help of others, being themselves fixed to other creatures which are actively locomotive.

Thus, sea-snails, lobsters, fishes, whales, and even ships, bear about with them sometimes lowly-organized plants; but often other animals, permanently fixed to and growing parasitically upon them and having the shape of their body suited to their peculiar situation.

Often such parasites form flattened encrustations on their involuntary hosts--as is the case with the acorn shells or sessile barnacles.[70] Others have elongated bodies, which stream through the water with the motions of the creatures carrying them. We see this in confervoid growths, also in ordinary barnacles, and in certain modified crab-like creatures, such as _Lerneocera_.[71]

These creatures fix themselves to their movable supports by means similar to those by which other creatures secure themselves to stationary supports. Thus, some of these do so by means of expanded disks, which fit accurately to the supporting surface, while certain parasites fix themselves by means of ingrowing prolongations or root-like processes, as in the _Rhizocephala_.[72] Others, again, adhere by the intervention of hooks and suckers, and this is especially the case with such as fix themselves internally and live perpetually bathed (as the tape-worms[73] do) in the nutritious fluids contained within the bowels of the creatures they infest.

Terrestrial mobile organisms can, of course, only be moved by their own efforts, or by the efforts of other organisms.

The simplest terrestrial locomotion is like that of the aquatic _Amoeba[74] primitiva_, and is performed by land _Amoebæ_; and the curious plant _Myxomycetes_[75] also moves in a substantially similar manner. This very curious organism consists of a net-work of protoplasmic threads, which spread over decaying leaves and stems. The threads exhibit streams of granules flowing within them, and they give out processes like pseudopodia, while the whole complex mass can slowly creep over a supporting surface, which it thus slowly flows over by its branching processes.

Other lowly plants propel themselves by means of a pair of filamentary protoplasmic threads, which vibrate actively, and are therefore called vibratile cilia. As an example may be mentioned the _Protococcus[76] nivalis_, the little spheroidal alga, which abounds on Alpine summits and in Arctic regions.

As in aquatic, so in terrestrial organisms, external form is intimately related to modes of motion. Thus, locomotion may be effected by undulations of the whole body, as often in serpents and terrestrial vermiform animals. It may, on the contrary, be effected by the action of levers projecting from the surface of the body, _i.e._, by limbs, and these may be multitudinous and minute, as in hundred legs and thousand legs, or few and large, as in beasts. Moreover, the motions may be movements of pulling or of pushing, or by combinations of these, or by jumps, which may be effected in various manners, the consideration of which will find a fitting place in an Essay devoted to "Motion."

Again, terrestrial, like aquatic, organisms often involuntarily carry about with them other living creatures which have fixed themselves to their bodies. Thus, the fruits, or seeds, of many plants (as, _e.g._, those of the common Agrimony, _Agrimonia eupatoria_) are beset with hooks or bristles which readily adhere to the coats of passing animals, and so gain a greater diffusion than they could otherwise obtain. A very remarkable form of the kind is _Martynia proboscidea_ (called Testa di _Quaglia_ by the Italians), which has a pair of curved and pointed processes like the tusks of an elephant, which are several inches long. It is notorious for adhering to clothes, &c. Other noteworthy plants are _Uncaria procumbeus_, or the grapple plant of South Africa and _Harpagophytum_,[77] the fruit of which is provided with hooked processes. Those of _Harpagophytum_ spread out in all directions, and are of different lengths, with sharp hooks, variously turned, so that its power of clinging is extreme. The seed, with all its processes, is so large as to fill the hand when grasped. It is said to cause the death of the lion. Having adhered to that beast's skin, the irritation produced and the impossibility of getting it off at last induces the lion to bite it, and once in his mouth he cannot remove it, and so the animal dies miserably.

Some animals fix themselves much as these seeds of plants do. Amongst them are the parasites known as tics which fix themselves with great tenacity by the appendages of their mouths. Other parasites--like the itch insect[78] and forms allied to it--have hooked processes and stiff, hard bristles, which are at once very irritating and very adherent. Creatures are also carried about inside others, as is the case with the seeds of many plants. These are disseminated by birds which have swallowed but have not digested such seeds, and in an analogous manner the great tape-worm group becomes also widely diffused.

Moving subterranean organisms, inasmuch as they must penetrate through a dense and highly-resisting substance, must evidently either have forms which offer little resistance--reducing friction to a minimum--or must be provided with special means of penetrating such substance. Evidently the least resisting form is presented by a body much elongated, rounded, and more or less attenuated at the advancing end, which end has to effect the requisite penetration. This is the form of the earth-worm--a form which is approximated to by a variety of creatures which have not the least affinity of nature with it, but only more or less resemble it as regards its dwelling-place and mode of locomotion.

Such, for example, are the curious serpents called _Typhlops_,[79] and such are the legless lizards[80] (_Anguis_), and such, again, are the simpler vermiform animals allied to frogs, called _Cæciliæ_.[81]

In order to burrow quickly and easily by means of processes of the body, it is evidently a necessary condition that the earth should be rapidly removed by the powerful action of parts situated towards the body's anterior end. The similarity of effect of similar conditions in creatures which are most widely divergent in nature is exemplified by the mole and the mole-cricket, which are each provided with a strong and broadened-out pair of anterior digging-limbs.

Living creatures may be sustained in the air for a longer or shorter time at one or another stage of their existence. The reproductive particles of the lowest forms of animals and plants are so excessively minute that they float in the air with the greatest ease, without needing any complication of structure--their spheroidal form harmonizing with the equal action upon them of influences on all sides of them. Reproductive parts which, though less minute than these, are still very small, may also be diffused by floating in the atmosphere. Such are the pollen grains of those trees which are fertilized merely by the action of the winds, such as the hazel, poplar, birch, and of lowly plants, as the grasses. It is by the wind that the pollen grains of these plants are accidentally brought into contact with the appropriate surfaces for their reception. Conspicuous in the spring of the year are the clouds of yellow dust, pollen grains, given off by fir trees, which are plants also wind-fertilized. But here we find a slight complication; for to facilitate the dispersion of such particles the outer coat of each of their pollen grains is produced into a short wing-like process on each side, and these processes help at once to sustain it in the air, and to aid its propulsion by offering more surface to the force of the aërial currents.

Very much more conspicuous are the wing-like expansions of many seeds--such, for example, as those of the maple. These expansions serve to diffuse the seeds which bear them, as do also the delicate cottony filaments which surround the seeds of a variety of plants of widely different natures and affinities, as some kinds of spider float through the air by the aid of the delicate filaments which they send forth to serve as an aërial float. Familiar to every one is the delicate little parachute-like structure of radiating filaments on the seeds of such plants as the dandelion--which seeds most children have at some time helped to diffuse by blowing.

Aërial progress by actual effort is effected by a limited group of organisms, and only in certain cases (bats, birds, and insects) does it take the form of true flight in creatures now existing. In other creatures, such as so-called flying fishes, squirrels, opossums, and the little flying dragon, the more or less prolonged aërial sustentation is effected by expansions of skin, which act as parachutes in ways be later described in detail.