Especially important in the easy and rapid locomotion of many unicellulars is the formation of fine hairlike processes at the surface of the body; in the broadest sense, they are called vibratory hairs. If only a few whiplike threads are formed, they are called whips (flagella); if many short ones, lashes (cilia). Flagelliform movement is found in some of the bacteria, but especially in the mastigophorous "whip-infusoria," in the mastigota among the protophyta, and the flagellata among the protozoa. As a rule, we have in these cases one or two (rarely more) long and very thin whip-shaped processes, starting from one pole of the long axis of the oval, round, or long cell-body. These whips (flagella) are set in vibratory motion (apparently often voluntary) in various ways, and serve not only for swimming or creeping, but also for feeling and securing food. Similar whip-cells (cellulæ flagellatæ) are also found very commonly in the body of tissue-animals, usually packed together in an extensive layer at the inner or outer surface (ciliated epithelium). If single cells are released from the group, they may live independently for some time, continuing their movements and resembling free infusoria. The same may be said of the travelling spores of many of the algæ, and of the most remarkable of all ciliated cells—the spermia or spermatozoa of plants and animals.
As a rule they are cone-shaped, having an oval or pear-shaped (though often also rod-shaped) head, which tapers into a long and thin thread. When their lively movements were first noticed in the male seminal fluid (each drop of which contains millions of them) two hundred years ago, they were thought to be real independent animalcules, like the infusoria, and so obtained their name of seed-animals (spermatozoa). It was a long time (sixty years ago) before we learned that they are detached glandular cells, which have the function of fertilizing the ovum. It was discovered at the same time that similar vibratory cells are found in many of the plants (algæ, mosses, and ferns). Many of the latter (for instance, the spermatozoids of the cycadea) have, instead of a few long whips, a number of short lashes (cilia), and resemble the more highly developed ciliated infusoria (ciliata).
The ciliary movement of the infusoria is held to be a more perfect form of vibratory movement, because the many short lashes found on them are used for different purposes, and have accordingly assumed different forms in the division of labor. Some of the cilia are used for running or swimming, others for grasping or touching, and so on. In social combinations we have the ciliated cells of the ciliated epithelium of the higher animals—for instance, in the lungs, nostrils, and oviducts of vertebrates.
In the unicellular, non-tissue forming protists, all the vital movements seem to be active functions of the plasm of the single cell; but in the histona, the multicellular tissue-forming organisms, they are the outcome of the combined movements of the many cells which compose the tissue. Careful anatomic study and experimental physiological scrutiny of the motor processes are, therefore, first directed, in the case of the histona, to clearing up the nature and activity of the special cells which compose the tissue, and then the structure and functions of the tissue itself. When we start from this point, and survey the manifold active motor phenomena of the histona as a whole, we see at once an essential agreement in the phoronomy of the two kingdoms of the metaphyta and metazoa, in the sense that at the lower stages the chemical and physical character of the motor processes can be clearly shown and can be traced to an interchange of energy in the plasm of the cells that make up the tissue. In the higher stages, however, we find striking differences, the voluntary character of many autonomous movements being very conspicuous in the higher animals, and thus the great problem of the freedom of the will is added to the purely physiological questions of stimulated movement, growth-movement, etc.
Moreover, the movements of the metazoa are much more varied and complicated than those of the metaphyta, in consequence of the higher differentiation of their sense-organs and the centralization of their nervous system. The former have generally free locomotion and the latter not. The special mechanism of the organs of movement is also very different in the two groups. In most of the metazoa the chief motor organs are the muscles, which have developed in the highest degree the power of definitely directed contraction and expansion. In most of the metaphyta, on the other hand, the chief part of the movements depend on the strain of the living plasm, or what is called the turgor or expansibility of the plant-cells. This is effected by the osmotic pressure of the internal cell-fluid and the elasticity of the cellulose wall, which is thus expanded. Nevertheless, in both cases—and in all "vital" phenomena—the real cause of the process is, in the ultimate analysis, the chemical play of energy in the active plasm.
The metaphyta, with few exceptions, are fixed in one spot for life, or only mobile for a short time when they are young. In this they resemble the lower metazoa, the sponges, polyps, corals, bryozoa, etc. They have not free locomotion. The motor phenomena which we find in them affect only special parts or organs. They are mostly reflex or paratonic, and due to external stimuli. Only a few of the higher plants exhibit autonomous or spontaneous movement, the stimulating cause of which is unknown to us, and which may be compared to the apparently voluntary actions of the higher animals. The lateral feather-leaves of an Indian butterfly flower (hedysarum gyrans) move in circles through the air, like a pair of arms swinging, without any external cause; they complete a circle in a couple of minutes. Variations in the intensity of light have no effect on them. Similar spontaneous movements of the leaves of several species of clover (trifolium) and sorrel (oxalis) are performed only in the dark, not in the light. The terminal leaf of the meadow-clover repeats its rotation, which describes more than one hundred and twenty degrees of an arc, every two to four hours. The mechanical cause of these spontaneous "variation movements" seems to lie in variations of expansibility.
Voluntary and autonomous turgescence-movements of this kind are only observed in a few of the higher plants, but stimulated movements that are accomplished by the same mechanism are very common in the vegetal world. We have, especially, the well-known "sleep," or nyktitropic movements, of many plants. Many leaves and flowers hold themselves vertically to the streaming rays of the sun. When darkness comes on they contract, and the calices of the flowers close. Many flowers are open for only a few hours a day. The mechanism of turgescence, which effects these swelling movements, consists in the co-operation of the osmotic pressure of the internal cell-fluid and the elasticity of the strained cell-membrane enclosing the cytoplasm. The strain of the outer cellulose membrane on the plasmatic primordial sac within it grows so much on the accession of osmotically active matter that the internal pressure is equal to several atmospheres, and the elastic strained membrane stretches from ten to twenty percent. When water is withdrawn again from one of these swollen or turgescent cells, the membrane contracts; the cell becomes smaller, and the tissue looser. Other stimuli besides light (heat, pressure, electricity) may produce these expansional variations, and, as a consequence of it, certain reflex movements (or paratonic variational movements). The most striking and familiar examples are the flesh-eating fly-trap (dionæa muscipula) and the sensitive plant (mimosa pudica); their contraction is caused by mechanical stimuli, shaking, pressure, or the touching of the leaves.
Most of the higher animals have the power of free and voluntary locomotion. It is, however, wanting in some of the lower classes, which spend the greater part of their life at the bottom of the water, like plants. Hence these were formerly held to be vegetable—thus the sponges, polyps, and corals among the cœlenteria. A number of classes of the cœlomaria have also adopted the stationary life, such as the bryozoa and the spirobranchia among the vermalia, many mussels (oysters, etc.), the actinia among the tunicates, the sea-lilies (crinoidea) among the echinoderms, and even highly organized articulate, such as the tube-worms (tubicolæ), among the annelids, and the crawling crabs (cirripedia), among the crustacea. All these stationary metazoa move freely in their youth, and swim about in the water as gastrulæ, or in some other larva form. They have taken only gradually to stationary habits, and have been considerably modified, and often greatly degenerated, in consequence; for instance, in the loss of the higher sense-organs, the bones, and even of the whole head. Arnold Lang has shown this very clearly in his excellent work on the influence of stationary life on animals. The study of these retrogressive metamorphoses is very important for the theory of progressive heredity and selection; it also shows the great value of free locomotion for the higher sensitive and intellectual development of the animals and man.
In many of the lower aquatic metazoa the surface of the body is covered with vibratory epithelium—that is to say, with a layer of skin-cells which bear either one long whip (flagellum) or several short lashes (cilia). Flagellated epithelium is especially found in the cnidaria and platodes; ciliated epithelium mostly in the vermalia and mollusca. As the lashing motion of these hairlike processes brings a constant stream of fresh water to the surface of the body, they first of all effect respiration through the skin. But in many of the smaller metazoa they also serve the purpose of locomotion, as in the gastræads, the turbellaria, the rotifera, the nemertina, and the young larvæ of many other metazoa. The vibratory apparatus reaches its highest development in the ctenophora. The extremely delicate and soft body of these gherkin-shaped cnidaria swims slowly in the water by means of the strokes of thousands of tiny oar-blades. They are arranged in eight longitudinal rows which stretch from the mouth to the opposite pole. Each oar-blade consists of the long hair-lashes of a group of epithelial cells glued together.