OF all the characteristics shown by animals sensitivity is the most apparent, whereas the growth of plants is their most obvious feature. The easiest way to tell if an animal is alive is to touch it, to see whether it will move in response to the touch. Similarly, the way in which a plant grows is an example of the same characteristic, I.e. the way in which it arranges its parts, as shown, for example, by placing two potted geraniums in a window and turning one pot through a right angle each day—the difference in the arrangement of theof the two plants soon becomes apparent. This sensitivity we term more strictly ’irritability ’and can define as ’the ability to respond to a stimulus ’enabling an organism to modify its activities for its own benefit or for that of the race, according to its surroundings, so that it is, to some extent at least, independent of them. In this respect, irritability is the most important of the characteristics of living matter.
The stimuli which produce responses in organisms include light, zcater, the force due to gravity, touch, the presence of chemical substances and, in animals, pressure and sound. The response made is generally a movement— either the whole organism moves from one place to another or some part of it moves. If the response made is towards a stimulus it is said to be positive ; if away, negative ; whereas if the organism arranges itself neutrally to the stimulus it is said to respond in a diatropic fashion.
Not all cases of movement are instances of response of protoplasm of plants to a stimulus, for certain non-living parts of plants are affected by changes in the environment. The effects may play an important part in the life history of the plants—chief among these are dispersal movements, e.g. the swaying of the poppy fruit in the wind.
This and similar examples malic quite clear the difference between irritability and the effect of a change in the environment upon non-living material—such effects we might term ’passive movements. ’ Since the organism has no control over such movements we can readily understand the advantage ofin producing such large numbers of fruits and , for the chances of their falling upon spots suitable for germination are slight.
Active Locomotion in Plants
Certain of the unicellular Algae live habitually in water and, unlike Protococcus, are motile, moving by means of cilia or flagella. Since they need light forit is necessary for them to swim towards light, I.e. they are positively phototactic. The ’male ’gametes of seaweeds, liverworts and mosses, and of ferns, are positively chemotactic—swimming when liberated towards some chemical substance secreted by the respective female parts of the organism.
Locomotion in Animals.
The simplest type of locomotion is that characteristic of Amoeba, where the cell membrane seems to show variable elasticity, the cytoplasm flowing into the bulges which appear where the membrane has temporarily become more elastic. Observation shows that the endoplasm is more fluid than the ectoplasm ; in fact its granules occasionally rebound from the ectoplasm when rapid movement is occurring.
In general, the animal responds by a positive taxis to useful stimuli, e.g. the presence of food and of oxygen, and by a negative taxis to harmful stimuli, e.g. excess of carbon dioxide in its environment, or by encysting should conditions become too unfavourable. It is interesting to note that the amoeboid cells in vertebrate blood show a positive taxis to those parasites which produce substancesto the tissues of the host.
We do not know how the stimulus causes the response in the organism, but it would seem to be due to some kind of conductivity in the cytoplasm itself, for the response begins by the formation of pseudopodia of a region farthest from the point of stimulation.
We have in this organism movement in its simplest form, for it is that of unspecialized protoplasm.
The cilia characteristic of the group of Protozoa to which Paramecium belongs are specialized portions of the pellicle covering the animal and contain clear ectoplasm. Their lashing movement occurs in quite a regular manner over the body, proceeding as waves from one end of the organism to the other, causing it to rotate forwards like a bullet. Contact with another solid body causes the cilia to act in the reverse direction so that the animal can only move itself forwards or backwards. Its rotation, however, sets up currents in the surrounding water, causing it to veer round somewhat each time it reverses. Here again there would seem to be some form of conductivity through the protoplasm, controlling the activity of the cilia in a regular fashion. The animal shows a notable response to the presence of substances in the water, e.g. acetic acid, causing the threads situated mainly at each end to be shot out, anchoring the animal against a surface or resulting in a ’hairy ’condition which is disliked by its enemies.
Despite the great complexity and specialization exhibited by vertebrates, their gametes show a reversion to a very primitive condition—the simple spherical egg and the spermatozoon, which consists of little more than a nucleus with a single cytoplasmic lash, the flagellum, which by its sinuous movement propels the tiny organism through its watery environment. Spermatozoa show positive chemotaxis to the substances produced by the eggs by swimming towards them.
Since it is one of the simplest of multicellular animals, it is not surprising to find that although certain cells in Hydra are specialized to bring about locomotion, these ’muscle-tail ’cells are not gathered together into one particular region, but lie scattered amongst all the others in both ectoderm and endoderm layers. The base only of each muscle-tail cell is contractile, arranged in the mesoglcea longitudinally if the cell forms part of the ectoderm, and tangcntially if part of the endodcrm.
Locomotion can occur in three ways, swimming, somersaulting and looping.
In forms higher than Hydra there are definite muscles, not scattered contractile cells. These in the earthworm are in layers. In this animal locomotion is rendered more efficient by the use of chastse for anchoring any part of the animal. They are extruded when their muscles contract, but when these muscles relax the chreta? lie unexposed, each in a little sac. Locomotion occurs as follows :—
Chffital muscles at the hinder end contract, extruding their cha^ta?, anchoring this end. The circular muscles anterior to this region contract; the pressure of the body fluid so set up causes the front part to be extended. Chajtal muscles at the anterior end now contract, extruding their ehaiUe, anchoring the anterior end, those at the other end being withdrawn, the longitudinal muscles contracting there to drag up that end. This sucoession of muscular contractions and relaxations is then repeated. Salt-water worms such as the ragworm Nereis have fleshy flaps on each segment with bundles of chsetse protruding from them. These are used like oars for swimming.
Locomotion in Higher Animals and in Insects
The higher forms of life have specialized blocks of muscles for moving particular parts, and some kind of skeleton for making movement more efficient.
In insects limbs are used as levers. Owing to the skeleton being external there is a definite limit to muscular development; but since these animals are small their muscles are extraordinarily efficient. Claws enable the animal to drag itself along rough surfaces, whilst many species have pads also for walking on smooth or wet surfaces. The wings are operated by flight muscles inside the thorax, the skeleton where the wings join the thorax being very flexible to allow for movement.
Fish have a relatively simple muscular system compared with that of insects, most of the highly streamlined trunk consisting of a repetition of similar vertebra; with muscles extending from one vertebra to the next. Locomotion is effected by the muscles on one side of the vertebral column being contracted, making that side concave. This sets up a sudden pressure of the tail upon the water on that side, the reaction of the water there moving the fish along the line of least resistance, which is head forwards. The muscles on the opposite side now contract as the first set relax, setting up similar results on that side. Unpaired fins help the animal to maintain an even keel, the tail fin is a rudder, whilst the paired fins are little used as organs of locomotion.
The other vertebrate forms, in addition to having their limbs adapted for locomotion, possess tendons connecting the muscles with the part they move, so that this part can be lightly built. Since the hind limbs are the main locomotor organs of land vertebrates, they are long, stoutly built and have claws or hoofs for gripping hard surfaces.
The long hind limbs of the frog with their powerful thigh and shin muscles are rather specialized locomotor organs, the webbing between the extraordinarily long toes enabling the feet to get a good grip on slippery muddy ground, and offering a large area to press against water when swimming, so that its reaction is very marked. The astragalus and calcaneum being very long act as an additional lever when leaping.
These animals are highly specialized for flight, exhibiting streamlined bodies, the wing feathers offering a sudden pressure to the air and being light, they are easily operated, whilst the whole breast region consists of flight muscles. Air sacs leading from the lungs, some even extending into the bones, reduce the weight of a bird considerably considering its size.
In mammals locomotion is rendered very effective by the tendency of those forms which run quickly to do so on the toes and not on the foot as a whole ; some of the side toes not reaching the ground are therefore useless for locomotion, e.g. the rabbit and hare have four digits, deer two, and the horse only one on each hind limb.