Locomotion of Animals

Rowing.

Fourthly, the man in the boat may row, the prin ciple being the simultaneous exertion of pressure on each side. Thus the platypus rows in the water with its webbed fore-feet, and the turtle with its paddles. Rowing in the air is the essence of flight in birds, bats and insects, though here part of the energy must be expended in keeping the body from sinking. Brittle stars or ophiuroids sometimes strike the sand with their posterior arms and may be said to row along the solid substratum, and in the mole's rapid turning in the ground the fore-limbs are used like oars as if the animal was rowing in the ground. The insect known as the water boatman (Notonecta) swims back-downward in the pool, using its long third pair of legs as oars. In many of the auk family the wings are used as well as the feet in swimming under water.

Other Methods.

The analogy of the man in the boat becomes somewhat forced when applied to out-of-the-way modes of animal locomotion, of which a few examples may be given. A jellyfish swims by alternately expanding and contracting the disc-like or bell-like body. The rapid contraction drives the water out from the mouth of the bell, and the medusa is propelled in the opposite direction. Cuttlefishes expand their mantle cavity, and having filled it with water proceed to close it by a hook-and-eye con trivance, so that the water cannot leave by the way it entered, but is forced, as the cavity contracts, through a narrow funnel. As the jet comes out with considerable strength, the body is driven rapidly through the water, with the head and tentacles in the wash. The same method of propulsion by a posterior outgush of water is seen in larval dragon-flies. Somewhat unusual, again, is the way in which lobsters jerk themselves tail-foremost in the water by suddenly flexing the posterior body (abdomen) forwards and downwards. This displaces a mass of water towards the head. On occasion the common scallop (Pecten) can jerk itself off the sea-floor with an energetic snap of its shell-valves, and continue swimming thus for some time.

In some ways the strangest mode of animal locomotion is exhibited by the common sea-urchin (Echinus) on a firm flat surface. This animal habitually moves by means of its tube-feet, and also utilizes its spines, which are swayed on ball-and-socket joints by basal muscles. But Echinus is also able to tumble along on the tips of the five teeth of Aristotle's lantern which project out of the mouth. The lantern can be swayed from side to side by powerful muscles, and the locomotion is a stumbling along on the tips of the teeth. The track shows at short intervals the indentations of the teeth, and marks of spines in between.

Mechanism of Locomotion.

The movements of most multi cellular animals are effected by muscles, and there is an important distinction between the unstriped or smooth muscle of sluggish animals, such as tapeworms, and the striped or striated muscle of ordinary active animals (see MUSCULAR SYSTEM), Unstriped muscle consists of homogeneous spindle-shaped cells, closely fitting, each with a single nucleus. Ordinary striped muscle is composed of elongated cross-striped cells, usually with many nuclei, which are sometimes situated peripherally, as in man sometimes embedded in the muscle-substance, as in the frog. A striped muscle-cell is usually a single cell; but in some cases it seems to be due to longitudinal fusion of several cells. The most

important general fact is that unstriped muscles contract slowly, and are therefore found in sluggish animals, and in the slowly moving parts of active animals, as the walls of the food-canal and arteries. Apart from these and similar exceptions, the muscle fibres in active animals are cross-striped and quickly contracting. Some of the lower animals such as turbellarian and nemertean worms are aided greatly by superficial cilia. The last occurrence of these superficial cilia as locomotor structures is in newly hatched tadpoles.

Among unicellular organisms locomotion is effected by flagella, by cilia, by myonemes and in an amoeboid fashion. Each flagel lum or cilium is a thread of protoplasm, sometimes with an axial filament. It is alternately flexed and straightened, as one might bend one's arm at the elbow and elongate it again. It is interest ing to notice that among multicellular animals cilia are very common, from the lowest to the highest, except in nematodes and arthropods, where the abundant chitin apparently precludes their development. As has been mentioned, the turbellarian and nemer tean worms are covered with cilia, which assist in locomotion ; but above the level of these two classes, cilia cease to be locomotor except in larval forms, like the trochospheres of marine annelids and molluscs. Starfishes and some other echinoderms are richly provided with external cilia, but these are used for wafting f ood particles, not for locomotion. Above nemerteans cilia become of great internal importance, for they may line a windpipe, an excretory tube, a female genital duct and so forth. Myonemes are contractile plasmic threads, anticipations of muscle-fibres, but intracellular. An instance of their occurrence is the axial fila ment inside the non-contractile sheath of the stalk of the bell animalcule (Vorticella).

Amoeboid movement, probably the most primitive mode of animal locomotion, has been much studied; but it is difficult to form a clear picture of what happens. An ordinary Amoeba, a naked blob of living matter, protrudes blunt finger-like processes and draws others in, continually altering its shape but not its volume. It glides along at the rate of about in. an hour. It is probably in some way gripping the substratum by a very delicate external plasma-membrane, which is continually giving way an teriorly, and being re-instated posteriorly. This film is the seat of surface-tension effects, and the tension seems to increase ante riorly and decrease posteriorly. But attentive scrutiny shows that the Amoeba is rather rolling than gliding. For a definitely recog nizable particle may be seen moving along the upper surface of the cell, in the direction in which the Amoeba is moving, then disap pearing over the front, and, after a while, re-appearing at the posterior end; and so da capo, as if there were a "caterpillar wheel"-like movement. Moreover, there is a deeper protoplasmic streaming, connected with intricate physical and chemical changes. There are indications of rapid changes of the protoplasm from "sol" to "gel" states and back again. It is very interesting to notice that this most primitive mode of locomotion is retained in varied expression even in higher animals, e.g., in the outgrowing tip of embryonic nerve-cells and in the excursions of phagocytes.